Publisher: Who is the publisher? Is it a university press? Is it publisher well-known for printing and distributing scholarly books like McGraw-Hill or SAGE? Visit the publisher's website to verify.
Author: What is the author's credentials or educational background? Is he or she recognized in her field of research? How many publications has that author produced?
References & Bibliography: A bibliography at the end of the book is a strong indicator of the book's contribution to scholarship, but be sure to note who exactly is being cited and their relevancy as well.
Content: How accurate and thorough is the content? Is there a bias? Look at the supporting material as well (graphs, charts, interviews, appendices, footnotes, etc.).
Primary vs. Secondary/Tertiary Sources: A primary source is an original source of work. Second (or tertiary) sources are reviews or overviews of previous research by other scholars.