Have you been in my living room, Naj?
Yup. 8 12x12x4 garden blocks and 4 12x4x1 plywood board (they'll cut them for you at Home Depot - just buy a slab and tell them the measurements). One can of paint and you have an instant bookshelf. I'd be lost without mine and refuse to part with it.
Oh I just go buy the pine boards, but you're right, the plywood is even cheaper. And you can arrange them anyway you want.
My "Bookcases" in my office are brackets on the walls and boards. Looks pretty cool to me!