Obviously, this question asks about reliable predictors for the quality of a book. Otherwise the answer can only be: You evaluate a book by reading it. With this out of the way:

• Ask around. In particular when you are interested in using a textbook for a course, ask people who have directed similar courses.
• Publishers. There is an informal hierarchy of publishers that can be used to predict a book's quality, just like conference venues and journal ranks for papers. This differs between disciplines, but the edge-cases are vanity presses (low-end) and Famous University Presses™ (high-end).
• Citations. Just like for journal articles you can count the citations of a book to estimate its impact (with all the known caveats).
• Reviews in journals. Many journals publish a review section. You can search these for discussions of the book that you are interested in.

Obviously, this question asks about reliable predictors for the quality of a book. Otherwise the answer can only be: You evaluate a book by reading it. With this out of the way:

• Ask around. In particular when you are interested in using a textbook for a course, ask people who have directed similar courses.
• Publishers. There is an informal hierarchy of publishers that can be used to predict a book's quality, just like conference venues and journal ranks for papers. This differs between disciplines, but the edge-cases are vanity presses (low-end) and Famous University Presses™ (high-end).
• Citations. Just like for journal articles you can count the citations of a book to estimate its impact (with all the known caveats).
• Reviews in journals. Many journals publish a review section. You can search these for discussions of the book that you are interested in.
• Peer review. Some publishers also send full book manuscripts or at least the book proposal with sample chapters out for peer-review.
• Professionals. Ask your trusted librarian for advice.