POST (Peace Officer Standards & Training) has a police leadership development reading list, but it's a bit anemic and even counterproductive in places. It includes The Killer Angels (which, while a great book, belongs on a soldier's reading list, not a cop's.).
It also includes (Gugh) On Killing but not On Combat. Therefore it keeps Grossman's thirty-year-old data that he still fucking preaches live AND his bullshit Sheepdog theory, but doesn't include On Combat's in-depth discussion of stress reactions and aftermath of a violent encounter.
Bare minimum, I'd add in Sebastian Junger's Tribe as well as Terry Pratchett's Watch series (Guards! Guards!, Jingo, Thud, Night Watch, and Snuff at the very least).