One more bit of advice if you want to be very thorough with this. Don't be afraid to ask your contractor questions throughout the process. They should be willing to give you a detailed plan of your requirements in a draft. Ask about sag and how they will eliminate it. Make sure they know exactly how much weight these custom built-ins would be holding at max capacity. You should have an idea of how you want to finish these shelves as well and if it's book friendly. I think stain should be fine, but I believe some types of paint CAN be a problem in regard to items placed on the shelf sticking to it. I don't think it will cause damage, but it's probably something you want to be careful with. Ask your contractor if they have advice in regard to finishes that work best on shelving. Never assume your contractor knows every answer or that they're experts on every facet of woodworking.