You also have a lot of books.
You can't put more books on the walls because the walls are full, or it's a small building (shed, treehouse), and space is at a premium.
on the ceiling.
This sounds silly. But the ceiling is one of the last unused spaces in a building, books don't take up space you'd otherwise be using if they're there.
Here is how to put books on your ceiling.
Step 1: Preparations
Wood, or other building material (you could use clear, hard plastic, or glass if you figured out how, to make it better).
Screws, nails, etc. An attaching thing.
Books (or cds, dvds, etc). Sorted basically by size.
Rafters, joists, etc make it easier to mount. If you're trying to find studs under sheetrock, it'll be harder.
Hammer (if you end up using nails).
Semi-exposed rafters, as you can see that I have, make this job pretty easy. Floor joists, as visible in a post-and-beam house, would work well as well. A flat ceiling will take a little more work, especially if you don't know where things can be securely fastened.