I heartily endorse reading the classic works of geometry as a way to both the subject as well as a way of thinking about math, proof, and argumentation. Start with Euclid's _Elements_, and then move onto Archimedes' short books on levers and floating bodies, Apollonius's wonderful treatise on conic sections, and Ptolemy's _Almagest_. They are excellent for self-education, providing both geometrical knowledge in itsel…
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.