Advice on book shelves

Howdy vSalonistas, need some advice on a book shelf I'm building for the wife. I'll likely build another just like it after this one is finished. She's got a TON of books.

Approx. Dimensions: 5ft tall, 4ft wide, 1ft deep

I'm building it out of 3/4" plywood with a red oak face frame to hide the edges of the plywood, and add stiffness. And that's where my question is. It's a wide book case, the shelves are 48" long. They'll be fixed to the sides, sitting in 1/8" dados and screwed to the sides with pocket screws. The backing is a 1/4" thick ply that's nailed in place, it's not really a load bearing piece.

I don't want these shelves to sag, but I also don't want to put a divider down the middle if I can avoid it.

So, I'm mounting a piece of (actual dimensions) 3/4" x 1.5" red oak along the front edge as part of the face frame, and to add stiffness. There will be lotsa pocket screws used to attach it. Will that be enough, or should I put another piece under the back edge of each shelf as well? Or should I bite the bullet and go ahead and put a divider down the center?

Basically a larger version of this record player stand I just finished:

http://i.imgur.com/RdeWJbY.jpg

http://i.imgur.com/cctZiC7.jpg

How about using a thicker backing with a dado to support the center shelf?

I built six book shelves this past winter out of 18mm baltic birch and they're bomber...only 38" wide, but super stout. The shelves have been loaded up since December and there's no sign of sagging yet and the shelves are only supported with pins (I needed them to be adjustable.) My backing is 12mm and inset into dados on each side of the case which was assembled using Festool dominos and a healthy amount of glue. Were I building a wider case and had a concern with sag, I'd cut a dado for it...but I'm not a woodworker so take it with a large dose of salt.

Put a beam under each shelf. Even 1/3 from the back will do.
Better a beam than a wonky shelf.

Paper is heavy, so are Vinyl records (ask me how do I know).
A 1.2mt flat span would need a lot of meat to stay unsupported. With 3/4" thickness available, would only work in square tubular steel.

Decided to change things up. Going to shorten the shelves to 40", but add a 2nd set with divider in the center. So effectively 80" shelves w/a divider/support at the mid point. Shelves sitting in dado's on the sides and the center support (so not relying on fasteners to support the weight). I'll still be using a 1x2 face frame on the front edge, and another under the back edge for additional stiffness. They'll be heavy as hell, but they'll be super stout, and give the wife enough space I won't have to build another one for a little while haha.

I'll post pics when it's done. Maybe this weekend...maybe next week.

http://i.imgur.com/H7swo6e.jpg

It's coming along. Books on top are to apply pressure to the center piece while the glue sets up, it sits in dado's on the bottom and top. I was SUPER stoked when I started putting it together in the office/bonus room, and I went to slide the shelves into place (they also sit in dado's on the sides and center wall) and they fit PERFECT. The fit is just loose enough you don't have to PRESS them into place, but you do have to keep them aligned perfectly straight or they jam up. Super happy with that. It's a pain working on something like this when you can get an hour two here and there, but I can see the light at the end of the tunnel at this point.

Also, it looks way bigger in the room than it did in the garage haha. I had to walk the top out to simulate moving it out, just to make sure we can get it out of the house once it's all put together. Not sure when it'll be 'finished', we're going to repaint the room before we move it into place and load it up with books.

http://i.imgur.com/f8R0bby.jpg

Well it took the better part of a year, but the Monster Bookcase is done. Pretty happy with how it turned out.

https://i.imgur.com/G40I0U0.jpg

Annoyingly after we got the room put together (for the first time since we moved in two Decembers ago) we realized that the built in shelves that were already there are pulling off the wall. Just another piece of shoddy work we've found at this house....