Normally, I'd recommend hardwood plywood, trimmed with solid wood of the same species. You get good yield at a reasonable price with minimal prep. Plywood will span less than most solid woods, though. An additional complication is that at 4'5" you are going to have a tough time getting a good yield from a 4'x8' sheet of plywood. If you could trim it down just a few inches, then you can get two shelves from an 8' length.

Given all that,
* 3/4" plywood for the case and shelves
* Trim front edges with 1/4" thick solid wood for flush trim, or 3/4" thick x 1.5" thick trim for a thicker look and more stiffening
* Front face frame of 3/4" x 2" cabinet facing, up to 3 or 4" for top and bottom. Depending on style, you can rout bead, ovolo, or round-over on the edges.

If you have a biscuit joiner, that is a quick and effective way to attach the shelves, top and bottom. Otherwise, rabbet and dado joints glued together. Or some other method such as plugged screws, Miller Dowels, etc. That is, joinery of choice.

Rabbet the back frame 3/8 x 3/8" and cut a piece of 1/4" plywood (same species) to tack on the back. This will greatly enhance the "stiffness" of the carcase.

I recommend finishing the inside of the case before putting on the back. You will find it easier if you do not have to do a three-way concave corner.

One defect to avoid is a void running right under the veneer. This will cause a problem getting an even finish and will create a weak spot that you can easily rupture.

Also avoid any Chinese import plywood. Any that I've seen is complete crap. Domestic, veneer core only for me.

It depends on what kind of tools you want to use, and how much you want to use each one. I could figure out a way to build a pretty solid bookshelf using only a circular saw and a drill because I hate using a table saw. But if, like many here, your table saw is like your third hand, then your options expand. In addition, do you want to be messing with glue and edging on plywood? Solid wood is far simpler.

It depends on what kind of wood you like. Personally I have very strong taste in wood and would pick based on what I like of what's available - wander the aisles at the lumberyard. And some woods are stronger than others, so the decision about what wood to use is connected to your preferred construction method. Warning: some are also heavier!

You are definitely looking at a substantial span there, and I too would worry about sag whether you use plywood or solid. The various bookshelves I have that aren't sagging (I didn't build them - and they're no where near your span) either have the shelves nailed to the backing (from behind - this is surprisingly effective even when the backing is only quarter-inch ply - or are at least 3/4 inch thick solid wood. If I have my physics right, foot-wide boards will be stiffer than narrower ones too. But by the way, I find narrower bookshelves far more useful; very few books need a foot of depth.

But your dimensions are a bit questionable in more ways than one. This is going to be a honking big piece of furniture that I wouldn't be able to navigate into some of the rooms in my house, much less set up in them. It would solve both your sag issue and their mobility to make two smaller units instead of one big one. Even if it's perfect for you at the proposed size, moving it or selling it will be torture - think transportation. Make two narrower units and they'll fit in any hatchback. The piece you're proposing will only fit in a truck. But whatever size you make, if you stick with that height, make sure you devise a way of fastening your unit(s) to the wall.

KarinL

Bookcases should have a finish that does not allow acid in wood to get to the books. Charles Webb used Sherwin Williams Super Chem-Var varnish, a catalysed finish that cures in 30 days, after which it has almost undetectable gas-off. It should be 3 to 4 mils thick. If tannic acid were released from the oak after finishing, the varnish layer would delaminate.

2" thick. Doubled up 3/4" birch ply with a 1/2" birch ply core and a poplar face frame. All paint-grade.

Overkill maybe, but no sag, strong enough to do chin-ups on them.

One screw in the center of a 4-foot shelf turns it into two 2-foot spans.

Strength varies directly with width.
Twice as wide, twice as strong.

It varies with the cube of the height (or thickness for a shelf).
Twice as thick, 8 times as strong.

The Sagulator takes everything into account (including the basic strength of the shelf material).

By adding a front edge twice as high as the shelf thickness of a decent strength material the shelf gets much stronger, as does fastening it to the back of the carcass.
If the back is thin it takes a lot of fasteners to spread the load, fewer with a thicker back.

Adding a single nail in the middle does not immediately cut the span in half though.

It might help to post a picture of what you hope to build. You should also start a new thread for this.