The American Historical Review publishes hundreds of reviews every year, and you can search them directly through JStor.
The New York Review of Books includes reviews from 1963 to the present.
ISIS covers scholarship on the history of science and includes book reviews.
H-NET (since 1995): An online source for scholarly reviews in the humanities and social sciences.
Internationale Bibliographie der Rezensionen wissenschaftlicher Literatur. An international index to book reviews in the humanities and social sciences. Each year is divided into 5 sections: A) authors reviewed, B) reviewers, C) titles, D) subjects, and E) periodicals covered. Instructions in English at the front of each volume.