Ask the builder: Building bookcases? Make a prototype first

Q: I live in a 100-year-old wooden house in Norway and am doing most the refurbishing and maintenance myself. I'm making floor-to-ceiling built-in bookcases for one of my living rooms and would love some tips. One of my challenges is the bookcase needs to span about 6 feet over a piano. What would you do to support those shelves so there's no sag from the weight of the books? Any information you can share that would make this project look magnificent and be trouble-free would be appreciated. &md