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.
White Bedroom with Bookshelves Hung High on Wall and Woven Rug. For even more storage, consider hanging shelves up high that run the length of the room. You won't loose any floor real estate at all, and you'll gain a lot of storage space.