Properly used, bookplates (also known as ex libris) can provide evidence of provenance, But bookplates, especially modern ones, also can easily be removed and/or forged. So unless you are well versed in the history of bookplates and their usage (including such items as mottoes, crests, heraldic arms, devices and the like) you might want to enlist the services of a specialist book dealer before making a decision to buy a book based solely on its bookplate. A good place to begin your education is with David Pearson's Provenance Research in Book History.
Nowadays bookplates are used less to establish provenance than to provide collectible specimens of decorative art, a trend that dates back to the mid-19th century. Because bookplate design has attracted significant artistic interest since at least the 16th century (including such notable artists as Albrecht Dürer, Hans Hoblein, Charles Ricketts and John Leighton), bookplates afford modern day collectors a relatively inexpensive means of building up a significant "miniature, personalized art-print collection:"
If interested in this aspect of bookplates, you probably will first want to examine one or more of the numerous books on this subject which have been published to date. These books cover both the general topic, as well as bookplates by specific artists. Examples include: American Bookplates: a Guide to Their Study with Examples; American Jewish Bookplates; British Bookplates: a Pictorial History; Historic California in Bookplates; Vinexlibris: Bookplates with Wine Motifs; and so forth.
There also are several blogs devoted to this subject. In the right-hand column, click on Confessions of a Bookplate Junkie under Bookish Blogroll for an example of same.
Numerous national organizations also exist for folks who are interested in bookplates as stand-alone collectibles. To find one in your part of the world, go to Bookish Organizations in the left-hand column and click on International Federation of Ex Libris Societies....
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