I really enjoyed this Antique Week newspaper article on collecting bookplates by Barbara Beem. She interviewed an expert in the bookplate collecting field, Lewis Jaffe -- here's the link to read it online.
The variety of images and subjects pictured on book plates is fascinating. The article has several illustrations, and you can see many more, and learn more, on Lewis's wonderful blog, Confessions of a Bookplate Junkie.
For more info, links, and a look at eBay listings, check out COLLECTOR WEEKLY's Bookplate Page. Ya gotta love CW -- they cover so many collectible topics, and so well!
They even have an article on bookplate collecting written by -- guess who? Lewis Jaffe.
More Bookplate history is in this British Museum book [Amazon]: EX LIBRIS
