Library Search is especially good for reviews of more recent books (and movies, plays, music, and games).
This database is a bit hit and miss on reviews. You are most likely to find some on more modern works, but I've also had luck sometimes finding full-text reviews of 19th century books.
A place to go to find reviews of books published during the fist three quarters of the 20th century.
Excerpts from and citations to reviews of adult and juvenile fiction and non-fiction from 1903 to 1982.