When determining whether a book is scholarly consider:
Publisher: Check the publisher's website. Is the publisher a University Press (e.g. Oxford University Press, Indiana University Press)? Does the publisher specialize in this academic field? Do they indicate their editorial policy?
Bias: Does the publisher have a religious or political affiliation? Consider how this affiliation might affect the scholarship and/or content of the book.
Authority: Who is the author? Do they have credentials that give them authority on the subject? Are they recognized by other scholars in the field?
Cited Sources: Scholarly books have cited references or a bibliography. Most books written for general audiences will not. Consider the quality of the sources: look for inclusion of journal articles, primary sources, and other scholarly books by experts in the field.
Content: Consider accuracy, bias, audience appropriateness, graphics/charts/illustrations. Look for books that have clear structure and organization, such as a preface, introduction, table of contents, conclusion, and index.
Still not sure? Look for book reviews of the book and see what reviewers say about its content.