They're rare, because the egg has to be incubated for a period of years, but as soon as it hatches, the larva needs access to books. This means the egg basically needs to be in a display case in a library, or in a museum that also has a book collection. Preferably in one with a dome on top, for some reason.
In the larval stage, it resembles a cranky librarian.
Over time, it gets scalier and crustier, and it develops a long tail, in which it wraps itself whilst thinking about books.
There was one at the Bodleian, at the Radcliffe Camera, whose name was "Yiddish," and it would sit on the desk, wrap itself in its tail, flash all its teeth at people, and then give advice. That was quite a young specimen. They get larger as they grow older, and more territorial.
Copyright libraries are examples of successful book-dragon hoarding. Under the copyright library, in the long-term stacks, the mature of the species is to be found, among enormous quantities of books that will never be read. The portion of the holdings accessible to readers is the bit the dragon puts out for bait; a successful library is able to build extensive collections.
The Vatican Library has a particularly old-established dragon with the most varied and valuable hoard in the world. This may be the alpha dragon of the species.
Sometimes the dragons get vicious. The Library of Alexandria had to be burned because the dragon whose hoard it was had gone bad, and there was no other way to contain the damage. The library of the Jewish Theological Seminary is ostensibly being torn down and replaced because of Manhattan real estate, but it's actually because they suspect a feral dragon, and the damage is less if you catch them early.
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