I've got a lot of big writing projects going on right now, and that is sapping my desire to post here.
But I did want to share one interesting link I found while working on one of them: Reflections on Relativity. It is a free, online book covering relativity (not just Einsteinian but also pre-Einstein conceptions), in a highly mathematical fashion that manages not to be so drenched in equations as to be incomprehensible without serious study, an extremely delicate balance.
I've only worked my way up to about page 58 of a ~600 page book, but even if I never make it past the first chapter I think it is worthwhile.
The book, at least to this point, has mostly just needed an understanding of calculus, but it is an intensely mathematical treatment in terms of how it approaches the subject methodically, building up from the bottom. Worth a try, if the subject interests you and the usual simplified-to-the-point-of-wrongness explanations are tiring.