Imagine that I am pushing on a wall. Then my textbook says that by Newton's third law, F A B = F B A FAB=FBA, where I am B and the wall is A. Isn't this wrong? Shouldn't it be that F A B = − F B A FAB=−FBA?

I believe they are talking about the magnitudes of the forces F_AB and F_BA. Per newton's 3rd law the forces are equal in magnitude and opposite in direction. So you are correct if F_AB and F_BA are vectors, not scalars.