I know a fair number of you want to shop at used book stores but find it intimidating and have a bad success rate on getting books worth reading, so I will tell you the secret:
The more cracked the spine of a book, the more people have read it completely through, cover to cover.
Buy the most cracked books you can find in any genre you like. They will be the most entertaining.
If the book isn't cracked, or is only cracked near the front cover, it's usually unreadable trash.