Werckmeister's appendix on keyboard tuning from his 1698 thorough-bass treatise:

Werckmeister_Anmerckungen_1698

Tuning instructions and other remarks from this appendix have been translated by Paul Poletti:

http://polettipiano.com/Pages/werckengpaul.html

Overview and discussion of sources for the French Temperament Ordinaire:

http://harpsichords.pbwiki.com/Temperament_Ordinaire

English and Italian 18th-century irregular harpsichord tuning instructions:

http://harpsichords.pbwiki.com/18th_C_Irregulars

excerpts from Neidhardt's writings:

http://harpsichords.pbwiki.com/f/Neidhardt_1724_ascii.html

http://harpsichords.pbwiki.com/f/Neidhardt_1724_sec2.1.pdf

http://harpsichords.pbwiki.com/f/Neidhardt_1732_ascii.html

http://harpsichords.pbwiki.com/f/Neidhardt_1732_sec6-8.pdf

Intervallic analysis charts for the 21 temperaments in Neidhardt's 1732:

http://harpsichords.pbwiki.com/f/Neidhardt_1732_Charts.pdf

Tiberius Cavallo's article on keyboard tuning in the Philosophical Transactions of 1788:

http://harpsichords.pbwiki.com/Cavallo

Thomas Young's description:

http://harpsichords.pbwiki.com/f/Young.html

Kirnberger's description of his temperament known as "III":

http://harpsichords.pbwiki.com/f/K_III.html

... and the letter to Forkel which is the only source thereof:

http://harpsichords.pbwiki.com/f/Kirn_1871.html