(This bookshelf is very sturdy and isn't going anywhere soon, so Please vote for this project in the Indestructables Contest in the upper right corner of the web page where it says, "Vote!")
I had located four dining room table leaves that were being given away through Craigslist for free (only the leaves without the tables). I did have to remove the end edges of the table leaves, as they weren't needed for this project. After deciding that I wanted to use the leaves as the shelves for the bookshelf, I set out looking for a way to accomplish that task.
During my research, a sturdy metal server rack became available at our hackerspace. It had previously held security monitors, printers, a computer, and a sound system. Those items had been relocated, freeing up the old server rack for a new purpose. I decided it would make an excellent base for holding up the table leaves as shelves, with the addition of some scrap 2x4's and plywood (We keep a stash of scrap lumber in our workshop). We eventually realized that the original metal shelves from the server rack could be attached to the sides as additional storage. They would also serve as bookends to the shelves, as it was decided they should be mounted offset to the wooden shelves.
If you can find an old server rack that is being thrown out, recycled, or for sale on the cheap, then you can build a bookshelf similar to this one. Since table leaves might be hard to find, but you could easily replace those with scrap lumber, 3/4" plywood, MDF, 2x4's, or even 1/2" slats. However, some additional reinforcement may be necessary if your replacement is not strong enough to hold books across the span of the shelf.
This instructable was an afterthought to the whole process, so I apologize for not having photos of every step.