Two books I would recommend, whether you are a trainer or a fitness enthusiast:
Advances in Functional Training, By Michael Boyle
This is a great book which covers a wide variety of exercises and training concepts.
Starting Strength, by Mark Ripptoe
Awesome book. He goes in depth on the Squat, Deadlift, Bench Press, Overhead Press and others. How to perform these exercises safely and even how to coach them.
Hope this is helpful!