There are three good printed OKR books:
All these OKR books are good, although different. John Doerr’s one is a good intro from the grandfather of OKRs. Radical Focus is a nice quick read in the form of a fable. Lamorte’s book is the most in-depth and theoretical.
As a free OKR e-book we suggest you to read:
Here is a more thorough review of the 4 books.