What's the best book for understanding Unity 2D?

There must be some guide which teaches the principles/ characteristics/ dynamics of Unity, and not just the basics like “how to print HELLO WORLD” or the steps to creating a game. A book that takes beginners to a more efficient path of learning and not just a prolonged trial and error process. For example I wish to understand how layers and cameras work, and the videos which I find on youtube or even the books I can find only show steps to doing something, but omit to explain why those steps are taken. Please don’t direct me to the Unity manuals. They tend to omit something here and there.