If you surf through the audioadvisor.com you might find offered wall mounts for the bookshelf speakers that look like a cross-bow. The resonance will be significantly reduced comparing to the book shelf.
Bookshelf Speakers
I am trying outfit a room with descent sound, but the only place I can put speakers is actually in a built in book shelf. I know it won't be optimal, but I really don't have a choice. Does anyone have any advice for speakers or set up tricks that might help optimize a not so perfect situation? Thanks. Greg