Thank you all for the feedback.
Yes, the bookcase wall is tall. 18' at the short end. The supports between are structural steel with their own footings.
The cases will be built in (6) 8' sections and (3) staggered sections for the bottom where they meet the lowest stair. They are entirely of plywood with fir facing on the verticals and 1/8" steel faces on the horizontals. All that will be seen of these boxes is the inside.
Keith, you said the following: I would make the dados 3/4" wide and stop one layer of the shelf the depth of the dado on each side. If you leave the sides wide you can dado and then rip to width after or take a batton and temp join then.
I interpret this to meen the following: You recommend making 3/4" wide dados for all the shelves, and extending only (1) layer of the (2) layer plywood shelves into the joint. Effectively a rabbeted shoulder joint. Is that correct? What is the advantage here? My only concern is that I would be required greater precision in all of my cross-cutting and gluing. I was even thinking of gluing full 4x8 sheets together with a vacuum press before cutting the shelves.
Additionally, it sounds like I am being encouraged to clamp/batten/secure two shelf sides together and cut a continuous dado across both pieces at once to guarantee alignment. That is a great idea. ripping the sides after cutting dados would be great to avoid blow-outs. I'll have to think about that. I was going to make an elaborate indicating jig on my worktable to align my home-made dado guide.
Thanks for all the input. This whole project has me very nervous, and now there is a big stack of expensive plywood in my shop just waiting for me to make mistakes.