I too have a 20+ year old pine bookcase I made with my first saw and
router. I used 1 x 12 no. 2 grade from a lumber yard. I used a beading
bit to put two grooves in the facing trim, a Roman ogee bit on the
shelves for accent and used v groove birch 1/4 inch plywood for the
back. It has been holding books for over 20 years and has endured
through several interstate moves and some abuse from the movers. About
all I had to do to firm it up after moving was to tap the facing back
down in 2 or 3 spots. Pine is fine.

If you use 1 bys buy as long a pieces as you can haul. They are
typically straighter than the shorter ones from shorter trees or tree
sections. You can sort through what is available at the lumber yard or
home center to get the straightest ones.

On my last pine project I found some pretty good 10 and 12 footers of 1
x 12 at Home Depot.

Good luck and enjoy it! You will probably have the book case for the
rest of your life.

RonT