This diagram shows the difference between the desk and the bookshelf - there really is minimal difference. It's all in shelf placement. Make sure you use 2x2s to trim out your desktop for a stronger desktop.

You will need to build two sides per bookshelf/desk. I show this diagram with pocket holes, if you use a Kreg Jig. While you can attach the legs to the side panel with standard screws (you'd want 2 1/2 to 3" long ones), I'd take a second to consider how much you are saving by building for yourself and invest in a Kreg Jig - it'll make your projects faster, easier, stronger.

Since 1x12s are notorious for being slightly different widths, take a measurement and cut 1x2 trim pieces to fit. Nail on with 1 1/4" finish nails and glue as shown above.

Next join the two sides on the top as shown above. One thing to consider when you use pocket holes is how to place your pocket holes for the strongest joint and to avoid splitting. Because the holes are pointed downward (as opposed to upward) in this instance, it will be much more difficult for you wood to split out on you. Always consider how you can place your screws in such a manner to create the strongest joint - but also in discreet locations.

For the bookshelf, build shelves, but this time, use 1x2s. If you are planning to place heavy objects on the shelves, use 2x2s instead (just like the bottom shelf). Attach to the sides.

In the final step, add your decorative feet. Follow the instructions given by the manufacturer.