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.
Step 2: Basic Idea
The books will be tipped.
One "stick" of wood goes under the bottom of the book.
One "stick" of wood goes along the spine (unless you have very tall books or wildly different size books in one case).
These pieces of wood are attached by secondary pieces of wood to a rafter, on both ends.
The rendered images are cutaways, and not actually how I attached it either, they just give the general idea.
Step 3: Begin
I was able to attach my bottom cradle piece directly to the rafters in a couple spots. You probably won't be able to.
You need to hold the book in place with one hand and with the other, measure how long your attachment pieces need to be.
Then cut and fasten them.
I ended up using nails to fasten to the rafters, as my drill was almost out of battery.
Step 4: The Cradles.
Step 5: The Books!
Now, this is kind of like an igloo - it only works when it's done. You should probably have all your books near at hand, because one book won't stay in the case by itself - it'll fall out. The bookcase needs to be full. Just hold one hand to the side of the books while you fill it.
In the previous incarnation of these bookcases, I used a weak bungee cord as a bookend, looped around the cradles.
Step 6: Additional Cases, Info
One of the great things about this is that you don't have to make them all the same way.
My first case, as you can see, has attachments on the insides of the rafters.
But after that, sometimes it makes more sense to put the attachment pieces on the outside of the rafters.
Or one each way.
Or attach directly to to another cradle.
I have one case with deep books in it, of varying heights.
It has two spinal cradles.
For a more finished look, also a look where the spinal cradle does not get in the way of reading the title of a book - maybe a short, wide piece of glass, or hard clear plastic?
Or instead of a 1x1 wooden stick, a thinner metal rod?
Attachments could be via chains/ropes if the ceiling is high! You could even have the cases set up so you could raise or lower them that way.