Several others have mentioned Guards! Guards!, and I want to echo that. That book and almost all the ones that follow in the Night Watch subseries (Men at Arms, Feet of Clay, etc.) have a "whodunit" element that your boyfriend might enjoy. Plus they're just phenomenal.

Mort is a great one, too! If he likes Death as a character and wants to keep following that thread, the next ones would be Reaper Man, Soul Music, Hogfather, and Thief of Time. The last two are two of my absolute favorites.