I wrote a whole post about swapping your bookcases for wall-mounted shelving. Wall-mounted shelving takes up less space, and can almost appear to blend in with the wall. Image from Stadshem, via Freshhome.
Floating bookshelves, these are bookshelves I like. Looks like you could do this with an IKEA kitchen wall cabinet minus door and turned horizontal to mount. IDEA for narrow wall next to our kitchen wall