When Phoebe look in the Book of Shadows, the demon is revealed to be Libris, a demon who attacks humans who have found proof of demons. However, later when Phoebe is in the library, Libris pulls her into the stack. Why would he do that after she's a good witch and she surely won't expose the world of magic. Shouldn't he be able to sense magical beings ? It's possible it was for his own self-preservation.