Bookshelves are an excellent way to use recycle wood -- especially since they support other items made increasingly from recycled wood: books.
Using recycled wood to make bookshelves can be very simple. If you have even-sized planks that are already sanded and finished, you won't even need any tools. You can simply separate them using other recycled materials (be it two concrete blocks standing on end between each shelf or anything else that will provide enough separation between planks for shelf space). In fact, you don't even need the wood to be sanded, painted or finished. You can use untreated pine boards or particle board for your bookshelf if you so choose. And why not, they'll be covered in books, after all.
Wood for bookshelves can even be recycled from older bookshelves. A piece that was used vertically on the side of a shelf (as opposed to the horizontal shelving) can be sawn into two or more pieces and used as horizontal shelving in the new project. Or old shelves can be inserted into a new support system.
Depending on the size and appearance of your recycled wood, you may need to make a plan, and figure out the ideal dimensions of your shelving, its vertical supports and its backboard (if you choose to have one). With the dimensions in mind, you can cut the wood to fit your needs, and then assemble it using nails, screws or shelf brackets. If you have extra pieces of wood lying around, you might want to use them as supports beneath the contact points where shelf-ends meet vertical support.
You'll need somewhere to sit while you read those books, which we'll take care of next.