Reposted response I wrote on another thread:

My general recommendation is this for people who are looking to improve their math abilities:

Study linear algebra, and real analysis. This is THE place to really begin mathematics. (edit: these are both prerequisites to properly approach calculus and differential equations)

These two fields will lay down the algebraic, analytic, and logical foundation from which to build. Once you've waddled through these waters, you will emerge a different thinker, with a new appreciation for mathematics, and you'll be able to guide your own learning path moving forward.

To this end, I will go a step further and recommend

www.amazon.com/dp/0387982582/?tag=stacko...20#reader_0387982582 (Chapters 1-5 most importantly), and

www.amazon.com/dp/0387950605/?tag=stackoverfl08-20
note: I am a mathematician first.