Tcl and the Tk Toolkit by J. Osterhout is an old reference but very useful for getting started with the C API to Tcl including embedding an interpreter. The last 1/3rd of the book is on writing Tcl applications in C.
Also look at the the source files for the Tcl (tclsh) or Tk (wish) shells. They are fairly concise.
Also look at the the source files for the Tcl (tclsh) or Tk (wish) shells. They are fairly concise.