…Genetic mutation from interbreeding, after the last global cataclysm. The people that survived repopulating the Earth. Not evolution. Just speculating aloud.
…These books may not be big enough, though.
…Random image search for 'big books'. Came from this link. https://medievalfragments.wordpress.com/2014/08/01/size-matters-part-2-giant-medieval-manuscripts/