If the bookshelf design is more traditional you might try movable cleats
supported on forward and aft upright strips. The uprights will have
corresponding raduis or 'sawtooth' cutouts every 2-3" to accept similar
shaped cleat ends. You can put a dowel in the bottom of the shelf, near
each end, to help contain the cleat in its vertical holder - a lot of them
didn't even use this. This was used in a lot of the old side-by-sides,
secretaries and other antique furniture and is pretty neat.

RonB