Andy wrote:
If you're worried about the shelves sagging, you might put a thin strip
of wood under the front of each shelf to help support the load. As far
as plywood, (and I'm sure someone will correct me if I'm wrong here) I
don't think Home-Depot-quality oak plywood would support any more
weight than decent pine plywood (excluding chipboard, particleboard,
etc.). I have some cheap assemble-it-yourself MDF bookshelves from
target or dumpsters or wherever, and they're only sagging a bit after
being fully loaded for several years. For the price, not bad.
My advice - try it with pine (start cheap), and if that really doesn't
hold your books, use it for attic shelving and go look for oak or
thicker pine for your next attempt. Have fun with it if you're doing
it for fun, or do it as quick and cheap as you can if you just want
something to set books on. Experiment with finishes on scrap pieces -
try sanding to different grits, sealer coats before stain, etc.
Andy

I can essentially guarantee that 3/4" pine shelves
8" wide by 48" long with a back of 1/4" plywood
(nailed into the back of the shelves) will not
sag. I made a bookcase like that with 3 shelves
about 1975, and for more than 20 years has has
held 8x11" magazines squeezed tight. There is no
evidence of sagging. Three similar shelf units
for my wife, but with shelves 12" deep, have been
loaded more normally for several years and show
no evidence of sag. BTW these are made of
Ponderosa pine which is pretty soft.

I would pick solid pine boards over fir plywood
(never heard of pine plywood) for a book case just
for appearance.