I am reading through Ahirman the Omnibus and not to spoil anything from the story but have come across a character who is essentially a possessed librarian. I feel as though in every other case whenever librarians let in creatures from the warp they die, are hideously mutated or are completely taken over by the daemon. In the book the relationship between the librarian and daemon is much more like Argul Tal and his daemon from the horus heresy. They form a team and work together arguably better then ether would have done on their own.
I am now definitely under the opinion that you should have the option to have a daemonically possessed librarian with better powers for chaos. Just wanted to see what everyone else thought.