What is a book about books? It covers any aspect of books and booklore—the love of books, favourite books, the history of books, the history of reading, re-readings, bookstores, reading festivals, book collectors, book thieves, banned books and great books.
You can find a list of the most often tagged books about books on Library Thing here.
I wish someone would invent an elegant word for the purpose. “Book about books” is so unwieldy.