I don't know if it has always been like that and I just noticed it now because I started playing with a headset, but the Black Books make some sounds when you're near them, similar to the Black Door. Mostly weird demonic growls and ominous music. They actually do what the Oghma Infinium should've done since it was clearly based on the Necronomicon, and I say that because the sounds are so intense that it gives the feeling that they are in the player's mind.
Really creepy and that made me hate the "dungeon-crawl" in Apocrypha 10x more 'cause now it REALLY feels like I'm facing everything from Cthulhu Mythos all at once.