It's been a while since Unity 4 was released, but new books were just trickling in. Now though, the floodgates have opened.
The following books have been added to the Unity book list:
Additionally, the following books were updated:
Professional Unity and C# (Cancelled)
Essential 3D Game Programming (Cancelled)
Unity 4.x Cookbook
Keep in mind, many of these books are in development. Some are a very long way away from release. As always, if I missed one or made an error, please let me know!
You can access the unity book list right here.