Wiktionary
a. (context idiomatic English) finished; concluded; able to be regarded as a matter of record.
Usage examples of "in the books".
You see, none of the creatures are exactly like the pictures in the books.
After four days, they had finally come near enough to the mountaintop so that Gar and Alea were willing to make a more or less permanent camp-but by this time, they were so deeply involved in discussing the ideas in the books that they left pitching camp to Blaize and Mira.
You didn't hand valuable territory to a competitor that close voluntarily, so Compworld had to have given something really major in return and nothing like that showed in the books.
He was there with the astronomer, come up from the cubbyhole where they'd been deep in the books from the Nantucket Athenaeum.
But I had to make a few littleahrightings in the books, because otherwise one who didnt understand the conditions might have drawn the wrong thocks from them.
Any similarity to characters or events in the books is purely coincidental.