Tcl and the Tk Toolkit by John Outsterhout, Addison Wesley, 1994
John Ousterhout's
paper
on Tcl from the 1990 Winter USENIX Conference.
John Ousterhout's
paper
on Tk from the 1991 Winter USENIX Conference.
The main distribution site. Info on where to get, and how to compile Tcl/Tk
is at Sun Labs
Where to get SpecTcl
Where to get blt
extensions needed for SpecTcl
The XF GUI builder