Don’t forget book reviews! Most databases will allow you to select book reviews as a source type. Reason: find out a book’s reputation, any hidden context, its significance among experts, and any problems with the book. Most authors are pretty persuasive and you need a second opinion.
Be sure to limit results to “review” or “book review,” depending on the interface.
New York Times archives is a good option for nonacademic books published before 2006.
JSTOR is another good one for many subjects.
Wilson OmniFile FullText. A good grabbag. You can limit it to search “book reviews” by a particular discipline.
These are not your only options but are some of the best. You can always check your favorite database to see if it has book reviews.