English language journals:
The Harvard Journal of Asiatic Studies and the Journal of Japanese Studies publish the most extensive reviews. Monumenta Nipponica has substantial reviews and is particularly useful for its reviews of books published in Europe often overlooked by other sources. All 3 journals are available online through JSTOR except for the most recent 2-5 years. Academic Search Premier will cover recent books reviews, but it does not include Monumenta Nipponica. Another alternative is to search Web of Science which includes all 3 journals, but is more cumbersome to search.
Japanese language journals:
Zasshi kiji sakuin indexes book reviews published in scholarly journals.