Hi Mark:
Nothing (I am familiar with) comes to mind that would sound good on a bookshelf.
Unless you find an efficient speaker designed for such an application I would not suggest putting much money into it (you would probably end up with an otherwise good speaker that does not perform well close to walls/boundaries).
An odd thought is to check out the Gallo DUE's which are designed for close to the wall placement (they can also be wall mounted). The online spec's do not state the ohm load, just efficiency @ 93 dB (which should be OK as they do not go that low).
