You can find some nice nude shelves these days. I have also seen some halfway decent finished ones at Lowes, of all places. Simple, but wood, not fibreboard that weighs a ton and warps.

I'm sure you know that any shelf over about 30" long isn't going to manage to hold books without sagging. I'm considering the trick of making super thick shelves by gluing two boards together and putting a nice edge on the front.

SRS