What sources did the author use?
Look for footnotes and/or a Bibliography (may be called Works Cited, Sources, etc.). This list shows you what kind of sources the author used to write the book.
What's the Purpose of the Book?
Scan the preface or the introduction for a statement on why the book was written and what the author hopes to accomplish with it. Often the author will also let you know what perspective of bias he or she brings to the topic.