This online bookstore has a massive range and categorises its books by subject or genre.
Many books have reader reviews to help gauge what the book is about and it's quality.
This site contains links to several review sites as well as links to many other sites relating to all aspects of books. This site covers to the fields of science, technology and medicine. Books are organised by subject and then by publication date. Each book is provided with a brief review. This site maintains a large and well organised collection of links to book review sites on the web.