Legion

It had great potential, but the pacing was much too slow. Instead of being a true horror story, it turned more into a study of the nature of good and evil.

This book, unlike the first, was like 80% murder mystery/detective story and only 20% demonic weirdness.
It still worked for me and even if they weren't related it would still work as this story. I read that this is technically like the script for the exorcist 3 and that I don't understand. This doesn't seem like it would work as a movie (I haven't seen them).

Also, the audio from audible was well performed.

Although not as good as it's predecessor, “The Exorcist”, “Legion”is a decent read in it's own right.
It's slow to start though, and the main character pontificates a lot. The final third makes up for this though.