The closest thing to a description of the C Language and compiler used for version 6 would be:
The C Programming Language by Kernighan and Ritchie. Note that this is the 1st edition describing the original K&R C; not later editions that described using the ANSI C language and subsequent revisions of the ISO C standards.
And note that, of course, the standard C library is not available in the kernel;
libc.a is only available to link with user level programs. And, if you're thinking about the shared libraries that are used today; they hadn't been envisaged (let alone implemented) in the UNIX version 6 timeframe.