At one point (I think about halfway through the third book) the angel/demon/forbidden/possible incest/doomdoomdoom/born to be evil/etc reached a fairly impressive level and I went "there is no way she hasn't read Angel Sanctuary." And then a character was reading it a chapter or two later. (The character was a 10-year-old boy. I'd be a bit worried by that, but his reading habits reflected his family's life pretty well.)