The Special and General Theory of Relativity - Albert Einstein
This e-book is the English Translation (published 1920) of Einstein's ground-breaking work, Special and General Relativity. Einstein is probably the most recognisable among all Physicists nowadays, and considered to be the greatest genius of all times.
Special and General Relativity can't be described better by anyone other than Einstein, their creator. With simple language and comprehensible text, this, according to me, is the best text of STR and GTR (abbreviations) that you can find...and isn't that obvious? :)
This is a public e-book for the United States. All other countries should check current laws about Copyright, although it is most probable that it is public there too, since 50+ years passed even after the death of the Author, let alone publish date.