Scholarly books are those published by university-based presses or presses which adhere to high-quality academic standards for the books they publish.
Examples include university presses such as Cambridge University Press, Stanford University Press, Nebraska University Press, etc.).
Examples of commercial scholarly presses include Sage, Elsevier, Springer, Wiley-Blackwell, Taylor & Francis, Kluwer, the American Psychological Association, and many more. If you're not sure if a publisher is scholarly, ask a librarian.