The following title is an annotated bibliography of book review sources. It is useful for finding obscure book reviews, including many in non-English-language sources.
A Guide to Book Review Citations: A Bibliography of Sources. Compiled by Richard A. Gray. Columbus: The Ohio State University Press, 1968. (Olin Reference Z 1035 .A1 G78)
Another approach to finding reviews not otherwise uncovered by the above resources is to enter the phrase books reviews in the Cornell Library's Catalog as a subject search. Browsing the list of subject headings generated by this search reveals many additional sources of book reviews. And remember that the reference staff is available to assist you.