I have written several books about Mac OS X (as it was called in the days) for publisher Van Duuren Media. The last one was about Mac OS X 10.5, Leopard. After that 10.6, Snow Leopard, came out, which was a 10.5 bug fix, and then iPhone came out. Suddenly Apple had a new toy to play with and Mac and MacOS (Server) played the lower fiddle at Apple. At least one can still run Ubuntu or Windows 10 on older Macs, you know, the ones that you can actually repair.