I'll try to respond, but I'm sure others will chime in with a better response. This is only my guess, but sure you can lay a bookshelf down and it will work. However, I think when a speaker is designed, the tweeter is on top for a reason. One being that there may be a design for the way the sound is directed horizontally into the room to achieve a good soundstage. By turning the speaker on its side, this orientation would change. Not sure how much negative effect you would have, and again I'm just guessing here.
I'm sure others will help out, I just think speakers are designed this way for a reason.
