Honestly, try your local library first. Check out a variety of what they have available. If will help you recognize the type of information that *you* find helpful so you can make a better purchasing decision. I've found that many of the "encyclopedia of fish" type books are great picture books but don't provide all that much in the way of real and useful information (like letting you know if a fish is a schooling fish and how many should be kept together to be happy, etc.).
The Baensch series is usually considered the best books for the hobby, but they can be pricey. If you have the time and patience, you can find them one book at a time on ebay or aquabid at a much cheaper price per book.
dmcl1's advice on looking at your local library is also very good advice. You could check out their books and if you find one you like, you can always buy it later online or at a used bookstore as Butterfly mentioned.