The books tag covers posts about books.

If you're looking for summaries and reviews, you might also find LessWrong's "Book reviews" tag relevant. On the Forum, the "collections and resources" tag might also be related. 

Some EA books have their own tag: The Precipice | Doing Good Better  | Poor Economics | Superintelligence | The Life You Can Save | Human Compatible  | What We Owe the Future