I was playing around with s2 a year or two ago, and it was surprisingly difficult to find information on it, so FWIW here are some links I was able to find at the time if you're looking for some historical reading on s2:

Google Presentation on S2: https://docs.google.com/presentation/d/1Hl4KapfAENAOf4gv-pSn...

Christian Perone's blog post on it: http://blog.christianperone.com/2015/08/googles-s2-geometry-...

Google Code Archive: https://code.google.com/archive/p/s2-geometry-library/

Java Implementation: https://github.com/rschreijer/s2-geometry-library-java