Occassionally one runs across reports of ToV observations before the telescopic era. Differences in the manner of stating evidence, observational contexts, and modes of transmission make it difficult to evaluate the reality of such observations from the context of our praxis of science. Those reports, even when inconclusive and intractable, are still of interest. Whithin the greater Isalmic cultural sphere there are ToV sightings — and doubts regarding their accuracy — in the period from al-Kindi (ca. 800-870 A.D.) to Ibn Bâjja (†1139 A.D.). The classic paper on these possible observations is Bernard Goldstein, Medieval Reports of Venus and Mercury Transits, Centaurus 14, 1 (1969), 49-59. It should be noted that Ptolemaic astronomy bequeathed to its medieval heirs — Islmaic, Christian, and Jewish — the tools to predict transits of Mercury and Venus. There have also been speculations that members of  First-Nations' communities may have observed ToVs. Trejo and Allen argue for the possible observation of ToVs by Mayan observers in the 12th or 13th century. There may or may not be extant traditions of Canadian First Nations' ToV observations, but if so, they have not been widely reported, or investigated.

A brief bibliography of secondary writings on ToV sightings before Horrocks' can be found on Rob van Gent's site.

Jeremiah Horrocks, Venus in Sole visa (Hevelius ed.), 1652

Jeremiah Horrocks, The Transit of Venus Across the Sun (Whatton ed.), 1859 — an epochal ToV work (Horrox's, not Whatton's), with some cringeworthy poetry from a century which produced better. Deficiencies in Whatton's translation have recently been noted by Mark Edwards, The Mistranslation of Jeremiah Horrocks’s Venus in Sole Visa, JRASC 106, 2 (April 2012), 61-69

Edmund Halley, De visibili conjunctione inferiorum planetarum cum sole, Philosophical Transactions 16 (1691) — at pp. 519-522 Halley gives his ToV predicitons, lays out the Halleian method for determining what we now call the Astronomical Unit (AU), and describes the necessary astronomical instruments with which to effect the recommended observations.

Chappe d'Auteroche et al., A Voyage to California, to Observe the Transit of Venus (Eng. trs.), 1778 (the French original can be consulted on the Gallica BnF site) — the narrative of one of the scientific martyrs of the ToV

Joseph-Nicolas Delisle, D'une Lettre de M. Delisle, écrite de Petersbourg le 24 Août 1743, & adressée à M. Cassini, Histoire de l'Académie Royale des Sciences. Année M.DCCXLIII, 419-428 — J.-N. Delisle, (Ch. Messier's teacher), presents his refinement of the Halleain method for determining what we now know as the AU. It is sometimes referred to in the literature as the Delislean method

Joseph Dymond and William Wales, Observations on the State of the Air, Winds, Weather, &c. Made at Prince of Wales's Fort, Philosophical Transactions 60 (1770)

James Ferguson, Astronomy Explained Upon Sir Isaac Newton's Principles... a Plain Method of Finding the Distances of All the Planets from the Sun, by the Transit of Venus Over the Sun's Disc, in the Year 1761: an Account of Mr. Horrox's Observation of the Transit of Venus in the Year 1639: and, of the Distances of all the Planets from the Sun, as Deduced from Observations of the Transit in the Year 1761, 1809

Edmund Halley, Methodus singularis qua solis parallaxis sive distantia a terra, ope veneris intra solem conspiciendae, tuto determinari poterit, Philosophical Transactions 29 (1715) — Halley's second paper on the ToV, refining and further developing the Halleian method for parallax determination. Halley suggests Port Nelson (latitude 56º) on Hudson Bay as a ToV station. An abbreviated English translation was published in 1809

Samuel Holland, Astronomical Observations, Philosophical Transactions 59 (1769)

Nevil Maskelyne, Instructions Relative to the Observation of the Ensuing Transit of the Planet Venus over the Sun's Disk, on the 3d of June 1769, The Nautical Almanac and Astronomical Ephemeris, for the Year 1769, Published by Order of the Commissioners of Longitude (1768) — instructions establishing the unofficial "official" procedures for observing the ToV, for observers under the British sphere of influence

James Short, The Observations of the Internal Contact of Venus with the Sun's Limb, in the Late Transit, Made in Different Places of Europe, Compared with the Time of the Same Contact Observed at the Cape of Good Hope, and the Parallax of the Sun from Thence Determined, Philosophical Transactions 52 (1761-1762) — Short's famous first attempt to synthesize those he deemed the best observations to derive a figure for the solar parallax. He discusses Winthrop's observations, but due to uncertainties about the longitude, decides against employing them. Short's second paper on the subject (Phil. Trans. 53 1763) can be found here

William Wales and Joseph Dymond, Astronomical Observations Made by Order of the Royal Society, at Prince of Wales's Fort, Philosophical Transactions 59 (1769)

William Wales, Journal of a Voyage, Made by Order of the Royal Society, to Churchill River, on the North-West Coast of Hudson's Bay; Of Thirteen Months Residence in That Country; and of the Voyage Back to England; In the Years 1768 and 1769, Philosophical Transactions 60 (1770)

John Winthrop, Extract of a Letter from Mr. John Winthrop, Professor of Mathematics in Cambridge, New England, to James Short, A. M., F. R. S., Philosophical Transactions 54 (1764)

John Winthrop, Observation of the Transit of Venus, June 6, 1761, at St. John's, Newfound-Land, Philosophical Transactions 54 (1764)

John Winthrop, Relation of a Voyage from Boston to Newfoundland, for the Observation of the Transit of Venus, June 6, 1761

John Winthrop, Two lectures on the Parallax and Distance of the Sun, as Deducible from the Transit of Venus, 1769

Thomas Wright, An Account of an Observation of the Transit of Venus, Made at Isle Coudre Near Quebec, Philosophical Transactions 59 (1769)

Transactions of the American Philosophical Society, held at Philadelphia, for Promoting Useful Knowledge, vol. 1 (1771) (also in Transactions of the American Philosophical Society, held at Philadelphia, for Promoting Useful Knowledge, vol. 1 2nd ed. corr. [1789])— this presents some of the "Canadian" results (Dymond, Holland, Wright, and Wales) along with those harvested by "American" and European efforts

Note: the Royal Society has generously made its Journal Archive permanently free to access. A great many papers relevant to the historical ToVs can be found in the Philosophical Transactions and successor publications. The Gallica BnF site hosts a multitude of historical and current ToV publications, including the Académie royale des sciences/Académie des sciences' Histoire de l'Académie royale des sciences ... avec les mémoires de mathématique & de physique... tirez des registres de cette Académie, and their Comptes rendus de l'Académie des sciences. Use "passage de Vénus" as a search term.

Sir George Biddell Airy, Account and Observations of the Transit of Venus 1874 Made Under the Authroity of the British Government, and the Reduction of the Observations, 1881

American ToV Commission, Papers relating to the transit of Venus in 1874, vol. 3, 1872

American ToV Commission, Instructions for Observing the Transit of Venus, 1874
This is reproduced here courtesy of the USNO's James Melville Gilliss Library. We thank Peter Abraham for his assistance in obtaining the document, and permission for its use

American ToV Commission, Observations of the Transit of Venus 1874, 1880

American ToV Commission, Instructions for Observing the Transit of Venus, 1882

Anon., The Canadian Almanac, and Repository of Useful Knowledge, for the Year 1882, Being the Second After Leap Year. Conatining Full and Authentic Commercial, Statistical, Astronomical, Departmental, Ecclesiastical, Educational, Financial, and General Information. The Astronomical Calculations Have Been Made Expressly for this Publication at the Magnetic Observatory in Toronto, 1882 — a predeccesor of the RASC's Observer's Handbook

British Government ToV Committee[?], Instructions to Observers, Nature July 20 1882 — these instructions were British adaptations of the "Instructions issued by the International Conference on the Transit of Venus, 1882", and were published in Nature as a means to disseminate them with dispatch to observers in the colonies

British Government ToV Committee, Report, Transit of Venus 1882, 1887

Charles Carpmael, Report of the Canadian Observations of the Transit of Venus, 6th December, 1882, 1882[?]/1883[?]

Alexander Johnson et al., Reports on the Transit of Venus 1882, in Proceedings and Transactions of the Royal Society of Canada I (1882-1883), 1883

George Forbes, The Transit of Venus, Nature Series 1874

Robert Grant, The Transit of Venus in 1874, 1874

Jospeh Morrison, The computation of the transits of Venus for the years 1874 and 1882, and of Mercury for the year 1878, for the earth generally and for several places in Canada, with a popular discussion of the sun's distance from the earth, and an appendix shewing the method of computing solar eclipses, 1873

Simon Newcomb, Discussion of Observations of the Transits of Venus in 1761 and 1769, Astronomical Papers Prepared For The Use Of The American Ephemeris And Nautical Almanac, vol. 2, no. 5 (1891)

E.C. Pickering, Observations of the Transit if Venus... Made at the Harvard College Obervatory, 1883

Charles Leeson Prince, Observations Upon the Late Great Comet and the Transit of Venus Made at Crowborough SSX, 1883

Richard Anthony Proctor, Transits of Venus, 1882

H.C. Russell, Observations of the Transit of Venus 1874 New South Wales, 1892

John Philip Sousa, The Transit of Venus, 1920 — according to Jay Passachoff this may be one of the worst novels ever written — definitely worth a look for those whose taste runs to cataclysmically bad art. Sousa's Transit of Venus March (1883) is generally considered more palatable; it's shorter.

James Francis Tennant, Report on the Preparations for, and Observations of the Transit of Venus, as Seen at Roorkee and Lahore 1874, 1877

only a sampling of reports on results from the 2004 ToV are listed here

Ashok Ambastha et al., Hα Observations of 8 June, 2004 Venus Transit, Solar Physics 233 (2006) — the authors make the claim that "... we present the results of the first ever observations of the transit made in Hα from USO [Udaipur Solar Observatory], Udaipur... This was also the first time that the transit was observed using a narrow passband Hα filter" pp. 172, 182. Unfortunately they are mistaken. The spectroscopists observing the 1874 and 1882 ToVs observed the transits at the Fraunhofer C line (=656.281nm=Hα line). For an example, see Captain G.L Tupman's observation in Account of Observations of The Transit of Venus, 1874, December 8, ed. Sir George Biddell Airy, 1881, p. 40

H. Boffin & R. West, The Venus Transit 2004 (VT-2004) Programme: The Exceptional Impact of a Unique Public Science Discovery Project, IAU Commission 55: Communicating Astronomy with the Public 2005 conference proceedings, Munich, 2005, ed. I. Robson & L. L. Christensen — a notable EPO effort, prefiguring the Astronomers Without Borders' Transit of Venus Project phone app project

P. Hedelt et al., Venus transit 2004: Illustrating the capability of exoplanet transmission spectroscopy, Astronomy & Astrophysics 533 (2011), A136

Jay  M. Pasachoff et al., High-resolution Satellite Imaging of the 2004 Transit of Venus and Asymmetries in the Cytherean Atmosphere, The Astronomical Journal 141 (2011), 4, article id. 112 — also see Pasachoff et al., The Black-Drop Effect Explained cited above
   
Paolo Tanga et al., The aureole of Venus: light refraction in the mesosphere during the solar transit of June 6, 2004, EPSC-DPS Joint Meeting 2011, held 2-7 October 2011 in Nantes, France

Several projects underway for the 2012 ToV are worth mentioning.

Dr. Alfred Vidal-Madjar has come up with a highly innovative project to use the Hubble Space Telescope (HST) to measure the transmission spectrum of Venus as it transits the Sun, to forecast the spectrographic signature of a telluric (Earth-like) exoplanet with a Cytherean atmosphere. The HST will measure the solar light reflected from the Moon during the transit. Dr. John C. McConnell of York University (Toronto) is part of this project's science team. A taste of the experiment design, modelling and expectations can be had through David Ehrenreich et al., Transmission Spectrum of Venus as a Transiting Exoplanet, Astronomy & Astrophysics 537 (2012), L2. Dr. Ehrenreich is the principal investigator.

In a related project, the Venus Twilight Experiment, led by Dr. Paolo Tanga, observers will make observations to characterize the measured brightness of the aureole through multi-band observations, using specially commissioned cytherographs (slides 24 & 25).  The Venus Twilight Experiment's results will also improve the characterization - and detectability — of the light transmission from Venus-like exoplanets.