If your shelf is currently pine, plywood will be stronger – I’d try “ItcheSrulik”s idea first.
Depending on your bookcase’s construction, you may be able to try one of the following ideas:
1) Attach a 2×4 to the back edge of the shelf for additional rigidity (a 1×3 or other piece of wood can also be used, but will add less strength). Go thru the top of the shelf into the short side of the 2×4.
2) Use an angled brace to support the shelf from the middle. This may only be feasible if you can screw into a stud behind the bookcase, since the backing itself usually doesn’t have much strength.
3) Use a double thickness shelf. You can use two layers of pine or plywood. If you glue and screw the layers together they will be stronger and more rigid.
4) Attach a piece of angled metal that’s used as a corner piece for metal shelving along the back edge of the shelf. (Sorry, I don’t know what this piece of metal is called, but it’s quite strong – I see it used to support air conditioners in the top of windows.)
There are 3/4″ and 1.5″ widths of molding sold in all shapes (half round, flat, beaded, etc.) that can conceal plywood edges, joints, etc.