Mathematics: Its Content, Methods and Meaning (Dover Books on Mathematics)
Also, you might want to get Richard Courant's "What is mathematics?" and a book (I'm reading multiple as I'm in your same spot, didn't have much mathematics during high-school because I thought I didn't possess the acumen, then I realized I really liked the subject) on proofs.
Gelfand's books are very very very good, trust me on this one, they build on the fundamentals. The books are not short of flaws though, namely the writing is informal, the author assumes some preexisting knowledge (that's why they are often not used as class books but as supplementary notes) and do not offer many exercises. But if you get the whole bunch you'll have covered the high-school curriculum (and more).
The AMA olympiad books are good reads, same with those "Art of Problem Solving" books. But personally, I'm not starting these until I've gained enough confidence, I still can't solve elaborate problems or mathematical olympiad kind of questions (but I'm getting smarter).
and this: http://www.amazon.com/dp/0486409163/?tag=stackoverfl08-20