An Infinitely Large Napkin [pdf]
Evan Chen is a very valuable person in the Math Olympiad scene.The Geometry book [1] he wrote in high school is required reading for any serious competitor and I believe he is the main responsible for some bringing some techniques into the olympic meta, such as Barycentric Coordinates.Also, his blog post [2] about writing is one of the best ones I've ever read.[1] https://web.evanchen.cc/geombook.html
[2] https://blog.evanchen.cc/2015/03/14/writing/