Make back rests(cleats) on the rear insides of the cases.

Add front supports under the front edges of each shelf.

That will allow the use of almost any 3/4" stock for the shelves and minimize any sag.

Hard maple 3/4" thickness is very firm.
Poplar 3/4" thickness Is slightly springier, but still acceptable.
Yellow pine 3/4" thickness is as firm as maple.
White pine (ponderosa from HD)is the springiest, but still within the acceptable range, just a bit weaker than poplar.
I used a 28" span, 30lb load (default) and 3/4" thickness at the Sagulator site.
A 28" span is not very much, and almost any common wood will be fine unless you're shelving hardcover books or bullion ingots.
Casey

Here is a link that might be useful: sagulator

Thank you, everyone.
That's the information I was hoping to get in just plain English.
Sombreuil_mongrel, that was my last calculator which made me a little crazy.

I can do exactly as advised and have yet another wall of cabinets together. Just gotta worry about doors!

Thank you all for your clear help!
Christine

Casey, I just reread your last line.

[whisper] Who told you about my ingots?

C.