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GrangalanJr · 2011-07-17
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Here are some nice introductory ones:
Mathematics: A Very Short Introduction (Tim Gowers): http://www.amazon.com/Mathematics-Short-Introduction-Timothy...
The Language of Mathematics: Making the Invisible Visible (Keith Devlin): http://www.amazon.com/Language-Mathematics-Making-Invisible-...
What is Mathematics?: An Elementary Approach to Ideas and Methods (Richard Courant): http://www.amazon.com/Mathematics-Elementary-Approach-Ideas-...
They give brief but interesting glimpses into several areas of higher math, so they could help you decide what you want to pursue in more depth.
Several authors I can recommend who write good expository books on mathematics to explain the why are
Ian Stewart
John Stillwell
Keith Devlin
William Dunham
Timothy Gowers has a book about mathematics in the Very Short Introduction series
http://www.amazon.com/Mathematics-Short-Introduction-Timothy...
that should be helpful to you.
On the particular subject you are studying right now, see if Hubbard and Hubbard's book Vector Calculus, Linear Algebra, and Differential Forms: A Unified Approach
http://matrixeditions.com/UnifiedApproach4th.html
helps your understanding.