Going to try do a 88" high 42" wide 15 deep book case in living room,one cabinet.Need help lifting haha no what I need to no is there going to be a sagging dilemma on account of the width. I will use all 3/4 inch plywood (Maple) and Maple hardwood facing on edges. Was thinking of 3/4 x 1.25 on shelving faces.
Thanks to all

Yes, you will likely encounter sagging over time. Go to This spot and scroll down to see ONE solution

Ront

Since the shelf unit is so large why not go with a two inch width board across the shelves' edges to reinforce and strengthen it?

42" is definitely too wide for any reasonable material. With fixed shelves you can use a cleat under each shelf in the center back and then a substantial facing on the shelves. Maple is a good choice.

If the backs of the shelves are attached to the back of the bookcase, that might be sufficient. If the shelves are adjustable, install the same under the back edges of the shelves.