Like shipping pallets, wooden cable spools are another item that is usually found for free on Craigslist. Since cable spools are limited by their shape there are fewer repurposing options, but they make a great coffee table bookcase.
Floating bookshelves, these are bookshelves I like. Looks like you could do this with an IKEA kitchen wall cabinet minus door and turned horizontal to mount. IDEA for narrow wall next to our kitchen wall