This is a mystery, in part, but truthfully, the whodunnit is not all that important in the end. I loved this book - the anti-hero (probably one of the first books I read where the protagonist was not really a good person), rare books, and the devil. It was really easy for me to get into. It's been awhile since I read it, but I still remember enjoying it (and really not liking the movie they made based on the book). 
 A warning - if you have a set belief about the Devil as a character (and do not want to read anything that deviates from that belief), then you might not like this book (which is fine, to each their own). The inclusion of certain devilish details was a surprise, but it didn't bother me. I really loved the ending and it made the book for me.