Richard Adams is probably my favorite author, though there are a few others that would be in the running. He is justly famous for Watership Down, which is by far his best-selling book, but some of his other works are just as brilliant, if not more so. If you haven't read any of his books, pick up a copy of Watership Down. You can find a copy at almost any used bookstore, or you can buy a new edition just about anywhere. If you've already read it, check out Maia or Shardik.