Using a router, you can build a free-standing set of shelves using only eight bits of wood: four legs and four shelves.
You need 18mm-thick sheets of wood for the shelving itself, whatever width and length you want. You also need four 70x40mm lengths of planed timber (preferably with rolled edges) for the legs; it is these bits that you need to cut slots into using the router.
Use a pair of clamps to hold the legs together, so that you can cut router slots through all the legs at once - giving a consistent cut and a much better chance of level shelves.
After routing, bash the shelving into the slots, drill a 3mm pilot hole into each joint (including the shelf wood), then open out the hole to 5mm in each leg and drill a small countersink. Fix it all together using 40 or 50mm long screws.
As you can see from the photos, the joint is very tidy and the shelves are a solid, practical affair. Don't forget to give them a varnish before they get piled with stuff!