Are books peer-reviewed?
Typically, after an author presents a book proposal to a publisher, the publisher will circulate the proposal to some selected reviewers to vet the content. This is not like peer review in the usual sense: the reviewers only get to see the outline and maybe a chapter or two. 

Once the publisher decides to go ahead with the book and it goes through the editing process, it might undergo further review, but nothing like a journal review.