I first read What Cops Know back in the late 90s when I was originally pondering the now-abandoned idea of a career in law enforcement. It informed a lot of my impressions of what police work was like at the time.
For those that haven't read it, it's a hard book to spoil, so I'd feel fine reading from here. The reason why it's hard to spoil, by the way, is that it's not one story, but a collection of lots and lots and lots of stories. Connie Fletcher (the author) basically took a bunch of Chicago PD guys out to dinner, threw a tape recorder on the table and said "tell me stories."
The stories the various officers tell range from the hilarious to the grim and bleak, and paint an interesting picture of what life is, or at least can be, like as a police officer in a large city. It also gives you an idea of what the officers are like themselves, as people. It's a fascinating character study in both police and the difficult members of society that they interact with on a daily basis.
Fletcher has laid out the book by topic and the stories are rarely more than a page or two, so it's the sort of book you can literally open to a random page and find something entertaining to read. Because of that, I've probably read it two or three times in chunks but never cover-to cover. It's also the sort of book that I've hauled around with me to read when I'm waiting on various things, a role in which it excels. If you read the book yourself and like it, she did two more in the same "Series" one called "Pure Cop" about specialized officers and one called "Breaking and Entering" about female officers.
It was a formative book for me in many ways. It presented an unromantic but not unappealing picture of law enforcement during a specific era (the 80s and little of the 90s) when I was thinking about getting into it and helped push me into getting my AAS in criminal justice. In retrospect, that was a huge waste of time, but in this case, the old saw about 20/20 hindsight applies, I think.