If you're not up for the do-it-yourself project but like the look of these tables made into shelves, check out Umbra's Biblioteca Bookshelf shown in images 4 and 5.
1. A brightly colored shelving unit from Camilla at Home .
2 - 3. Designer Isabel Quiroga's bookcase made of old tables, called "Storyteller", via Treehugger
4 - 5. Biblioteca Bookshelf in a lovely room shown on Desire to Inspire.
(Images: as credited above)