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.