I see what your saying. I'm renting so I'm not going to be able to build anything. The windows are about 30 inches from the floor. Any other suggestions?

You can place several book cases end-to-end to fill whatever space you need. Just be sure to measure the area from the floor to below the window ledge to know exactly how much height you have. Then take the tape measure with you to the store to make sure the item will fit where you want it. It's much better to keep shelves to 36" to prevent sagging from the weight of the books. HERE is a longer unfinished birch plywood bookcase that solved the support problem by putting in extra support partitions.

Some of the IKEA Besta units, including their entertainment units, might work as bookcases/storage units.

http://www.ikea.com/us/en/catalog/products/00275848/

kallax shelf unit inexpensive and very nice looking

You can "build" with the idea of taking it with you..... Room and Board and make a base if needed.MULTIPLES of sizes available in multiple finishes

http://www.roomandboard.com/catalog/living/bookcases-and-shelves/rollins-stacking-cubes

This is my favorite book shelf style for small spaces it is called Sapien from DWR it comes in 3 heights