Dr. Bradley Lehman offers a keyboard temperament analyzer/calculator, along with cogent explanations of the need for temperament. Lehman has also recently published a radically new interpretation of Bach's well temperament.
Tuning & Temperament Bibliography - a huge resource with many links
Temperament Offsets: Researched by Willis Miller for Avery Todd, an exhaustive list of offsets for historical temperaments.
Understanding Temperaments: Excellent introduction to historic temperaments.
The Well-Tempered Clavier: Fictional, strangely entertaining and instructive account of all Bach's Preludes and Fugues, including recordings.
Music History timeline
"Historical Tunings on the Modern Concert Grand" -- an excellent discussion and exposition by piano technician Edward Foote, whose chart was my original inspiration for creating this website.
Alternate Temperaments: Theory and Philosophy (The beginner's field guide to alternate temperaments)
The Open Music Project: links to alternative tuning systems
Temperament: A Beginner's Guide, by Stephen Bicknell
An Introduction to Historical Tunings, by Kyle Gann