The Elements

I will join with anatoly in disagreeing. The Elements are a beautiful piece of literature and I recommend reading them for a historical understanding. (I have not read them through by any means, but I have read segments which are magnificent). As for understanding mathematics and its techniques, there are now much better methods. Our understanding of mathematics and methods of learning (not to mention our simple culture in general) have changed substantially since then, the most obvious point being the development of non-Euclidean geometry. To add to the list of books already mentioned consider Numeracy. It was influential for me when I was an undergraduate and touches on numerous topics. Also, if you want to start at the beginniner, there is a very good book called Chapter Zero that starts with nothing and is easily accessible to anyone willing to study it.