Books usually count as academic sources, but it depends on what kind of book. Textbooks, encyclopedias, and books published for commercial audiences often do not count as academic.
Consider these questions when you're deciding if a book is academic or not:
- Who is the author? Google him or her. The author should be an expert in the topic of the book with graduate degrees and preferably a current position at a research institution like an university.
- Where does the information come from? There should be lots of references and other evidence in the book to support the arguments or findings.
- Who is the book written for? The book should not be written for laymen.
- Who published it? Academic book publishers are often university presses, like Oxford University Press, but you will encounter other academic publishers, like Routledge, Palgrave or the American Psychological Association.
If you're still not sure, the best thing to do would be to ask your instructor or a librarian for confirmation.