I've done a lot of reading as I research the type of mystery novels I want to write. With police and lawyer stories at one end and cozy mysteries at the other, my favorites are somewhere in the middle, stories with characters who have a variety of interests, who can grow and develop into someone you'd like to know. I also like when they are set in a specific locale that became intrinsic to the plot.
Here's a great NHPR interview with Brendan DuBois, the author of the Lewis Cole mystery series that I have read over the years. Reading his books helped me figure out the type of mystery I want to write.