I don't think the word "more" is there for any grammatical, syntactical, or lexical purpose. It is there to emphasize the state of disarray in the house. This can be seen from various sentences in the preceding paragraphs:
There were piles upon piles of books and papers on every surface.
"Excuse me," said Xenophilius, and he strode over to the machine, seized a grubby tablecloth from beneath an immense number of books and papers, which all tumbled onto the floor, and threw it over the press, somewhat muffling the loud bangs and clatters.
He glanced vaguely around the room, at the piles of parchment and books, but Hermione said, "I've got a copy, Mr. Lovegood, I've got it right here."
Thus, the author is saying that in addition to all the books and papers strewn about mentioned earlier, there were yet more books present.