This Animal probably feeds upon the Paper and covers of Books, and perforates in them several small round holes, finding, perhaps, a convenient nourishment in those hulks of Hemp and Flax, which have pass'd through so many scourings, washings, dressings and dryings, as the parts of old Paper must necessarily have suffer'd; the digestive faculty, it seems, of these little creatures being able yet further to work upon those stubborn parts, and reduce them into another form.This picture came from the Project Gutenberg's eBook of Hooke's work, which you can read in full online. There are more resources about bookworms: William Blades's 1888 Enemies of Books devotes a chapter to giving a heated account of the damage bookworms can do (it's also at Project Gutenberg, or you can find it here). More scientific, and less entertaining (albeit probably more useful) is the information given in the Encyclopedia of Library and Information Sciences:
Bookworms in the larval stage of a variety of beetles cause the most damage. Upon hatching, the larvae eat their way into the book, whose glue and starches nourish them. The well-fed larvae become beetles, lay eggs, and recommence the cycle.That's a bit dry, to my taste, but there is useful information about killing the critters with napthalene fumes. They also explain what you might be wondering: why are there plenty of wormholes in old books, but not in newer ones? Our methods of making paper today are not as tasty. Mmm, china clay!