If your book collection is threatening to take over every last inch of your shelving, perhaps it is time for a major review of how you display your tomes.
Start by emptying your shelves. It's easier to figure out what you want to do with them when you have a blank slate.
Remove the books you no longer want and donate them to local charities. This will help your shelves remain clutter-free and leave space for new books you really do want to read.
Decide how you want to categorise. Popular choices should be in alphabetical order, by author or title, or by genre.
Alphabetical listings are the most obvious organisational method, and probably the most reliable. You can follow the library system and arrange by the author's last name, or by title. Remember 'the' and 'a' don't count. After they are organised, be sure to double-check to make sure they are in the correct order.
If you have lots of different types of books, it may be easier to arrange by genre, such as fiction, non-fiction, literature, biographies, coffee table books, or whatever it is you're interested in.