Placing bookshelf speakers on a bookshelf

As close to the edge as possible and use brass cones or black diamond racing cones to help isolate the speaker from its own vibration.

Audioadvisor has much better solution for your case is to use wall mounts that can adjust any angle of radiation. Check out for this inexpencive solution.

I have been in this situation for the last 15 years or so (extremely playful cats). Place the speakers centered on the Left/Right portion on the shelf, as close to the front of the shelf as possible. If the speakers are rear ported, experiment with plugging the rear ports, as well. Do you need to place the speakers on their sides ... if so, the woofers would be on the inside walls of the wall units, the tweeters would be on the far side. The sound will be OK to pretty good. If the wall units are 5 foot tall or taller, do not place the speakers on top of the units ... you loose too much.
Rich