One old crate
Some matching boards
A couple of nails
One obliging brother
Step 1) Find an old crate. Mine came from my Dad's garage.
Step 2) Convince your brother to put a shelf in said crate.
Step 3) Sit on a stool in the garage and watch said brother build a shelf in crate.
Step 4) Say "Thanks, you're amazing."
Step 5) Carry bookcase/crate to bedroom and fill with books.