There was some papers and a book (sorry can't remember anymore which one, there used to be more out there about TCL) I used but mostly looked at the expect source as an example.
Also look at the the source files for the Tcl (tclsh) or Tk (wish) shells. They are fairly concise.
Although even that is not super recent.
Tcl's C API is very pleasant to work with, and its C code is a good read - except for the regex stuff, which are the first few files you'd see in an alphabetical listing.