I got a terrible education before University, I didn't even know what a theorem was, all math was just "learn how to do it, understanding will not have you pass the exams", I'm not having a bad time in college, but some of my colleagues (who got a strong math background) know things that I not even imagine that existed, and I feel offset about this.
Things Like Geometry for example, I love it, I understand what I'm doing, but some basic concepts I just accept as truth and don't know from where this result came from.
I would like to know some textbooks that would fill this gap (not only in Geometry, but in all basics), I'm not a freshman, so maybe something more rigorous will be interesting.