Presumably the ones from the library, which the author mentions was his source? Every Pratchett book I read as a kid matched this description, including being battered.
Tell me more about this already-guilty-looking paper, and how this kid was "sliding" an entire paperback book into a math textbook with "a centimetre to spare".
I dont know what kind of pockets you have, but most Pratchetts book did not fit into mine. And yes there were whole series of books that fit. But, pratchetts ones did not.