Space, Time and Gravitation by Sir Arthur Eddington, completed at Distributed Proofreaders (DP) in 2009, is an account of general relativity from 1920. Eddington led one of two expeditions to observe the total solar eclipse of May, 1919. The astronomical goal was to measure deflection of starlight by the sun’s gravitation; Einstein predicted a minute but theoretically measurable deflection twice as large as Newton. The photographic plates notably conformed more accurately to general relativity than to Newtonian gravitation. You can read a bit more about the book and its historical context at the DP Blog.
(Independent experiments in subsequent decades have demonstrated general relativity to be incontrovertibly more accurate than Newtonian gravitation, though the differences are not apparent to unaided human senses. Today, general relativity governs the design of particle accelerators, describes bending of light by massive astronomical objects, and explains clock drift in global positioning system (GPS) satellites with exquisite precision.)
Also by Eddington, The Mathematical Theory of Relativity, first published in 1923, is a technical exposition. Starting from experiential concepts of distance and duration, Eddington introduces the mathematics of tensors, (what we now call) connection coefficients, and covariant derivatives, then uses these mathematical tools to describe gravitation, mechanics, and electricity. (I am not sufficiently knowledgeable to compare Eddington’s account with more recent and standard treatments.)
On a personal note, these books represent endpoints of my time digitizing for PG. Space, Time and Gravitation was among the first group of LaTeX books I post-processed (performed final assembly, document-level coding, and consistency checks) at DP. The Mathematical Theory of Relativity is the most recent book I solo-produced, in 2019, just months after the book entered the public domain in the US (publication date plus 95 years).
Digitization of books in LaTeX for PG is skilled, meticulous work, requiring about 20–60 minutes per page overall, depending on the amount of mathematics. In the aughts and teens, and probably today, optical character recognition software made hash of mathematics; all but the simplest mathematics had to be keyed manually, then scrutinized for correctness. At 240 pages (excluding front matter and the index), many of them math-heavy, The Mathematical Theory of Relativity conservatively represents 100 hours of volunteer work, excluding the time required to scan the original for the Internet Archive. Although unpaid, the work can also be gratifying (pride of typographical craft, for example) and enjoyable (getting to read books with care to the extent digitization allows).
Many, many dedicated volunteers around the globe help create repositories of no-cost, fact-based, high-quality educational materials available to everyone with an internet connection. While you have an internet connection, these materials are there for you, gifts from the past to the unknown future we all build day by day with our words and actions.