At the moment, in February of 1998, I am aware of two good sites.
The first is The Bryn Mawr Review site, and the search engine is accessible here. The BMR provides reviews of books in classical and medieval studies, and searching for the word "democracy" will produce a long list of works touching on Greek and specifically Athenian democracy.
The second is the H-Net Review Site. H-Net is a consortium of discussion lists and other resources for the whole field of history, and so its search site will give you access to many reviews in many different areas.
If you know of additional review archives that would be of interest to other students of democracy, please let me know.