The big problem seems to be the assorted wires, devices, and similar, that you can't stack very easily. I would recommend putting them in a box or boxes, so you can stack other things on top of them, and it's easier to put them somewhere else (under the bed, say), without too much hassle, and without makes things untidy.
I would also recommend putting any loose papers in a ring-binder if they can fit vertically on a shelf, or a box file, which can stack, or go vertically. Smaller manuals could also go into box files for protection, and to keep related ones together.
Basically, if everything is stackable and easy to move, that should help you use the space more efficiently, and make it easier to rearrange. Also, make sure stacks are level. I don't like the look of the book underneath the black box on top of the shelves, it could make the box unstable if you put anything else on top of the box.
Those are the main things I think you could do.