Fantastic Fiction is a website that is always first on my favorites. Here, you can search by title and author, browse books, or find books similar to one you really liked.
It's my go-to place for finding out what books are in a series. This is of ultimate importance because, assuming the series is complete, I won't start a series until I have all the books that belong with it. I really do not like being left hanging.
If you love books, want to find out information on authors, or just find a new book to read, this website is a necessity.