Answer:
I have a mouth on my head and eat everything.
I have a straight back and sharp teeth to cut objects.
I wiggle and cannot see; sometimes underground and sometimes on a tree. I really don't want to be on a hook and I become a person when combined with “book”.
So simple, that I can only point. Yet I guide men all over the world.
I can be simple or I can be complex. I can be found in this riddle or in everyday life. I can be shapes or even colors.
No matter how little or how much you use me, you change me every month.