Fantastic Fiction - This site, based in the United Kingdom, lists bibliographies for more than 30,000 authors. It's great for finding book placement in a series and associating authors with all of their pseudonyms. The site can be browsed or searched by author or title.
What Should I Read Next? - This site provides instant recommendations of individual book titles when you search by title or author. Results are fine-grained enough to recommend a different set of books when selecting different books by the same author. The site isn't extensive, but it's growing and provides long lists of recommendations for most popular titles and authors.
Goodreads - This site is best used to find a variety of reader reviews for an individual book, so it's great when someone asks "will I like this?" It lists a short number of suggested books in a sidebar on each title entry. Fair warning, the site features lots of advertising.
Juvenile Series and Sequels - This site is created and maintained by the Mid-Continent Public Library and contains an indexed list of more than 3,700 series. The database can be browsed by series title, series subject, book title or author. Even better, results are displayed with labels designating Easy, Juvenile or Young Adult.
Reading Group Guides - This site is maintained by the group that produces the Bookreporter (also a good source of book reviews). The best ready-reference information can be found under the "Find a guide" tab. This provides lists of most popular titles for book groups and books clustered by theme, as well as indices of books and authors. Each title entry contains a book description as well as suggested discussion questions -- useful for both book groups and reader's advisory.