A lot of people reading this have probably heard the phrase "bookworm." For those of you who haven't, one, you're probably living under a rock, and two, it means someone who really likes books - a bibliophile. They are most often depicted as nerdy little worms of some sort, often comically reading a random book. So are bookworms a real, actual species of insect?
Answer: Yes and no. There are a number of insects and invertebrates that could be called 'bookworms.' Silverfish and cockroaches are quite fond of hiding in between pages of old books and taking a bite. There are bugs called "booklice" which are not really lice at all, instead being flying insects that have been around since the Permian. Even rodents occasionally chew on books. Some of them are interested in the glue; others tunnel through the pages like wood; rodents will gnaw on anything to keep their teeth filed down. Really, especially in the modern age, a book is a very safe hiding place with some food lying around. There's no reason for bugs not to invade such a place.
All sorts of things can eat books, but the term 'bookworm' usually refers to the larvae of one of two beetle species: the common furniture beetle (Anobium punctatum) and the death watch beetle (Xestobium rufovillosum). Even then, these two are usually more interested in the wood around the books than the book itself. Books are made from trees just like wood, and so happen to be on the menu. They are like junk food to these baby beetles.
On that note, take bookworms seriously, especially if the larvae in question belong to the common furniture beetle. Beetles called "bookworms" will fly in through any little crevice and lay their eggs near books. These are, however, wood-boring beetles; any wood that isn't made from "heartwood" (the inner wood of the tree) is prone to infestation, as is any untreated wood.
The most obvious signs of bookworms are, of course, little larvae and tunnels in your books. Look for 'dust' on your bookshelves if you think your wood might be infested. To prevent them to begin with, keep your books in a clean, dry place. If you think you have a whole library's worth of little grubs, there are a few chemicals, including tobacco, that you can use to fumigate the area. Do your homework before trying to get rid of other bookworms.
Although some of the cartoony bookworm characters are quite benign and adorable, this is definitely not the case in real life. Real bookworms can wreck an entire library without checking out anything. Nature sees paper as "very thin, dead trees." Why shouldn't stuff take advantage of it?