I've been thinking of a series of books I read when I was a teenager and I'd like to find them again!
It's about a guy who captures demons with a flute or a pipe, something like a wind instrument.
I remember he started off in college with his best friend trying to summon a demon and it of course goes wrong... he ends up joining the demon and the soul of his best friend together and this best friend pops up in the books as a great character sometimes.
I remember in one of the books the lead guy has his jaw ripped off by a demon and can't play his flute anymore, I think they are in a stairwell.
I remember it was a cool set of stories.
I think in one book (not sure if I'm blending books together now) he is hired to investigate a haunting of a museum or library where rare books are kept. Everyone thinks the place is being haunted by a nun with a red veil, turns out it's a woman that was killed by the janitor by punching her in the face whilst holding a set of keys and that's why her face is red when she's seen in the building. I remember they plot out all the sightings of her and it's like she's tied on a string to the basement apartment.