The City Watch stories are where I would start. It's some of the best writing, the best character in Sam Vimes, some of the best individual stories (Nightwatch), better positions the secondary characters like Death in the Discworld and also murder mystery as your boyfriend likes.

• Guards! Guards!
• Men At Arms
• Feet of Clay
• Jingo
• The Fifth Elephant
• Night Watch
• Thud!
• Snuff