Schopenhauer's life can only be described as a dark, sad, long and lonely, and he'd finish each day by reading a few lines of the Upanishads, so his thoughts on reading and books ought to be taken with a big lump of salt, but here they are anyway:
Similis simili gaudet - 'Birds of a feather flock together.'
Repetitio est mater studiorum - 'Repetition is the mother of studies.'
"We can never read the bad too little and the good too often" works for me, and I wished that it were true that "to buy books would be a good thing if we could also buy the time to read them". Danke schön, Arthur!