blueman wrote:

I am building a built-in bookcase with a 48" shelf span and want to
make sure that I am designing the shelving system properly to support
the span with minimal deflection. The shelves need to hold heavy
textbooks.

- The sides and back of the bookcase are 3/4" cabinet-grade birch
plywood.
- Plan is to create 48"x12"x1" shelves by gluing together two sheets of
1/2" birch plywood.
- Shelves will be held in-place by a pair of 1/4" shelf pins at each
end. I plan to use sleeves to reinforce the shelf-pin holes.
....

I personally think it will be marginal w/ ply. I'd make every attempt
to find solid lumber to make the shelves from. It would take some
clearance away, but you could use an edge on the shelves to gain some
extra stiffness if you could live w/o having close margins between tops
of books and next shelf. Overall, for a span over 3-ft, I'd really try
to put in a vertical center support if I could.

The 1/4" end pins are probably marginal and w/o the inserts I definitely
think they will tend to round if a 4-ft shelf is loaded to the maximum
possible. There are library systems that are more sturdy. Seems like
Woodworker Supply had some??? Or was it Woodworkers' Hardware in MN??? I
can't recall just now....

If it were me, I'd split it I think w/ single carcase, center support.