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.
《I really like how the cases on the end are slightly recessed from the rest, but the molding on top is a bit much. Raise bookshelf heights & use same molding & molding line as the rest of the room for a much nicer look》built-ins