For those who prefer to learn on their own, Noumena recommends TclTutor and Tcl/Tk: A Developer's Guide.
Tcl is the Open Source movement's biggest hidden success story. It runs the telecom industry from the EDA tools used to design the chips to the test platforms that validate them up to the applications that run on Cisco Routers and AOL's web server.
The language supports modern constructs like catch/throw error handling, encapsulation, modularization and Object Oriented. It's fast and easy to learn, and powerful enough for the US DoD and NBC Broadcasting studios.
My book's goal is to bring a programmer up to speed on Tcl/Tk as quickly as possible. I describe the basics of the Tcl/Tk language, techniques for using it effectively, and tools for helping you develop applications better and faster.
A few Barnes & Noble's Bookstores keep Tcl/Tk: A Developer's Guide in stock. If your local book dealer doesn't stock it, feel free to tell them that they should stock my book. :-)