Readers interested in learning more about Tcl and Tk are encouraged to followup this article with some of the resources listed below:
Tcl comes with an excellent online manual system. Tcl and the Tcl shell, tclsh, are described with "man Tcl" and "man tclsh" respectively. All commands that are part of the Tcl base environment are described in the "n" section of the manual. Readers interested in learning more about optional parameters to Tcl commands are encouraged to make use of the "options" man page.
John Ousterhaut's "Tcl and the Tk Toolkit" is in its 14th printing, and remains a classic programming text. In addition to providing the basics of Tcl and Tk scripting, "Tcl and the Tk Toolkit" also provides a great deal of information about how to extend Tcl and Tk in the C programming language.
Brent Welch's "Practical Programming in Tcl and Tk" is now in its fourth edition and is a very supplementary good reference to the Ousterhaut book.
comp.os.lang.tcl is the primary newsgroup for people using Tcl and Tk.
Scriptics.com is the primary online resource for the Tcl/Tk developer community.
Cameron Laird maintains an excellent online resource of Tcl and Tk tutorials at http://phaseit.net/claird/comp.lang.tcl/tcl_tutorials.html.