Joe, is what you want the distressed look? Like a piece is really old?
There are a lot of ways to do this. The most basic way is to add wear marks with things like rasps on edges, whacking with a chain with big links, sanding with number 1 grit sandpaper (a big rock with a lot of corners). One thing for sure is that it is easy to overdo distressing, so don't go too wild initially.
Since you are building a bookcase, you will have a problem doing much of this on plywood. Usually, a back will not see much distress but the top, shelves and sides will see a lot. The edges of plywood do not take well to distressing with veneer taped edges.
When you have created the wear areas, you can finish. You will want to sand the worst of the mechanical distressing that you did usually. Just smooth the rough edges. As far as a finish. you can stain and varnish or use other techniques. I have done some finishes with a stain and them paint and the paint sanded away on corners and other wear areas. Looks neat. You can also wipe on a little Minwax Special Walnut in a few places to simulate "dirty" paint.
You can also duplicate peeling paint areas by globbing on a little rubber cement before painting. When the paint is dry, use your finger or sandpaper to rub the paint and rubber cement off. Again, it's easy to overdo this.
It's usually best with finishing tricks like this to put on a clear top finish to seal the surface. And usually this is one place where a little amber hue from oil based poly adds to the look.
The problem with all these techniques is that you want to consider what you want the piece to look like, how distressed, and then figure how you want to get there. I can't stress too much that you want to go a little on the easy side with these techniques until you have some experience. At the same time, these are fun to do. You do also want to be sure that you will be using materials that will not have averse reactions together.