Here is a list of all documented files with brief descriptions:

src
libexpr-c
nix_api_expr.h	Main entry for the libexpr C bindings
nix_api_external.h	Libexpr C bindings dealing with external values
nix_api_value.h	Libexpr C bindings dealing with values
libfetchers-c
nix_api_fetchers.h	Main entry for the libfetchers C bindings
libflake-c
nix_api_flake.h	Main entry for the libflake C bindings
libmain-c
nix_api_main.h	Main entry for the libmain C bindings
libstore-c
nix_api_store.h	Main entry for the libstore C bindings
libutil-c
nix_api_util.h	Main entry for the libutil C bindings