Duane Bozarth writes:
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.

What type of solid wood (without knotw) would you recommend that I
could buy in 12" width, 1" thickness, 4 (or 8) ft length without
breaking the bank?

I don't have a jointer so I would prefer not to have glue up a panel
using narrower stock.