Einstein, "On the electrodynamics of moving bodies" https://www.fourmilab.ch/etexts/einstein/specrel/www https://www.fourmilab.ch/etexts/einstein/specrel/www

Einstein, "On the electrodynamics of moving bodies" https://www.fourmilab.ch/etexts/einstein/specrel/www https://www.fourmilab.ch/etexts/einstein/specrel/www It's remarkably accessible and clear (at least Part I is).