What a great question! Here's a few of my favorites:
1. Animal Underworld: Inside America's Black Market for Rare and Exotic Species - This book is a bit dated (1999), but it offers a fascinating firsthand account into the exotic wildlife market in the US. It gives great context to the current situation of captive-bred wildlife and the differing legal systems at the time regarding ownership.
2. Forbidden Creatures: Inside the World of Animal Smuggling and Exotic Pets - This one was also a really good read. The bit about pythons was really interesting.
3. Stolen World - This was a fascinating book on rivalries in species collecters, and provided great context for the current reptile market in the US.
4. Killing for Profit - This book exclusively looks at the illegal rhino horn trade. I haven't read it, but I've been meaning to.
5. Animal Investigators: How the World's First Wildlife Forensics Lab is Solving Crimes and Saving Endangered Species - This book was a great introduction to how forensics are being used in combatting wildlife crime.
6. Rewilding the World: Dispatches from the Conservation Revolution - Not particularly crime related, but it shows different avenues that are being taken to restore species and populations, which can be involved in recovering populations threatened by wildlife crime.
I'll add more as I think of them, but this should be a good start.