These shelves fall precisely into the "nomadic" furniture tradition proudly pursued by fellow Instructable-ers. Since the flats have handles on them already, and a nice rim around the edges, the idea is that the shelves can break apart and be used as containers for your books while moving.
This is a really simple project that can be done with a screw gun and a hand saw if really necessary, but a bandsaw or jigsaw will really speed things up.
Step 1: Structure
The base of the shelves is one flat that has had the handles trimmed off flush with the rim so that it would lie evenly on the floor. Then, two 3-1/2" by 3/4" notches were cut in the bottom of the flat, one on each side, shown as the grey patches in the first drawing. A Dremel with a metal cut-off wheel or RotoZip works great for cutting the plastic. To take the burrs off, run a scrap of 100 grit sandpaper over the cut edges.
Cut your wood as shown to whatever height you want the shelves to be spaced apart. Fit the narrow projection of one of the pieces of wood into the notch, and screw through the side of the flat to secure it. I used cheap drywall screws. Pre-drill holes, especially if you are using MDF, to prevent splitting. Repeat on opposite side.
Cut into another flat the same way, and align it above the first, so that the notches are above one another. Screw in place. That makes a complete base unit. For the stackable, modular shelves, sucessive pieces only have one flat and two piece of wood. See the second drawing for how they are supposed to fit together.
An eighth or quarter-inch diameter threaded rod spaces and braces the back.