Donald Coxeter. (Amazon reviews here.) This would be an awesome, awesome book to add to a late-high-school math course, with the exception of a few strange pages about his adventures in psychoanalysis, and a bit of profanity near the end. I keep coming across things that I'd like to quote here, then I keep reading and forget to post them.
Please understand that I have absolutely no qualifications for reading or understanding this book. I have no post-secondary education in mathematics of any stripe, geometry or otherwise (although I did briefly work in a university math office). In fact, I'm pretty sure that my high school mathematics completely skipped over even the basics of what Coxeter spent his life doing, and we certainly NEVER talked about the historical rift between geometry and algebra. (See Nicolas Bourbaki...who didn't exist.) Which is really sad, because it might have helped make a little more sense of what we did do.
But I'm enjoying it anyway.