I wrote C in the 1990s on IBM mainframes, PC/DOS, and OS/2. I used some IBM mainframe, Microsoft, and OS/2 documentation. At one time I wrote some C code callable from PL/I for the TCP/IP calls, available to C but not to PL/I. I wrote some little utilities in C on OS/2, e.g., for sorting files of strings. Recently I wrote a grand, very carefully considered solver for Ax = b where A is a m x n matrix and x and b are m x 1. So, the A need not be square and if square need not be non-singular. I was fairly careful about numerical accuracy, etc.

For the C documentation, I recall only one point: Due to all the different addressing options on x86, the Microsoft C compiler had a crucial but nearly secret command line switch we needed. I found the switch only by some strange efforts. It was not in the documentation. None of the documentation I had was much beyond just K&R.