In article ,
(Jay Chan) wrote:

I'd use 3/4" for the sides and shelves, 1/2" for the back and doors. Do
no tuse particleboard of MDF as it will be prone to sag.
...
Polyurethane.

Thanks for the good advices. I definitely will not use particleboards.
I got too many sagging book shelfs that were made from particleboards
that I bought when I was in college.

Jay Chan

I bought some cheap utility shelving units from the Borg a few years
back. The frame is snap-together angle iron, with particle board
shelves. The 18" deep units seem to be OK, but the 24" deep ones sagged
horribly. I ended up cutting some 1x2 strips and screwing them to the
bottom of the shelves for extra support. Pretty ugly, but good enough
for utility storage in my basement.