The British Library Digital Catalogue of Illuminated Manuscripts: An online catalog created by the British Library giving access to one of the most significant collections of medieval manuscripts on the web. Easy to use for the novice as well as the scholar.
Pecia: This site by Jean-Luc Deuffic is the only blog dedicated to the study of illuminated medieval manuscripts. Deuffic writes in French but has recently started providing English translations for his entries. Check out what he has to say about different manuscript topics and practice your French at the same time! "Pecia" means "piece" in Latin and is the name for a system used for copying texts in the Middle Ages.
The Medieval Bestseller: The Book of Hours
The Medieval Bestseller (Getty Exhibitions): Gives an overview of the most popular type of book of the Middle Ages -- the book of hours -- using examples from the collection of the J. Paul Getty Museum in Santa Monica, CA.
CHD Tutorial on Books of Hours: Want to know more about the most popular type of book -- the "bestseller" of the Middle Ages? This site gives the nuts and bolts of the contents of these books and how they were used. Created by the Institute for Studies of Illuminated Manuscripts in Denmark.
Hypertext Book of Hours: This site by Glenn Gunhouse offers the opportunity to see the contents of a typical book of hours in both Latin and English translations.