Think about it - it's a wooden box that you then attach a back to, and throw shelves in. The difference in all of them is how you decide to construct them, how you "dress 'em up", and how wide you want to make them to suit your needs.
You can do hardwood sides like Scott suggested, or plywood sides with a face frame (for lower cost)
For the shelves, you can route dadoes in the carcase sides, route a dado and install tracks for track shelving (not pretty and not what I'd suggest, but PERFECT for quick and easy projects), or make a jig and drill holes for shelf pins (many others, but those are the "typical" methods)
For the "dressing up" part, you can use your router and a variety of bits to create your own moldings, or you can use off-the-shelf premade moldings and combine them to create a different look.
For shelves, you can use solid wood, you can use plywood with a solid wood facing strip, you can use plywood with the face frame idea above, you could use 1x stock and thin plywood for a torsion box type of shelf to support more weight
IMO, a bookcase is a super nice and easy project for someone to get their hands wet, and when you move up in skill, you can look over in the corner at that first bookcase you built that is now being used to store tools in or some other such use and see how you've progressed.
ANYONE can make a wooden box with a back on it. Slap shelves on it, and they call it a bookcase. Go figure!
