The fourth edition of Book Practical Programming in Tcl and Tk
About about 20 sample chapters from the four editions are available online . They cover topics from the basic Tcl and Tk introductions, through more advanced topics like Regular Expressions, [eval], socket programming, thread programming, TclHttpd, and starkits.
Among its other virtues, the fourth edition explicitly explains use of Tk by such other languages as Python, Ruby, Perl, and so on.
2008-10-22 : I wonder if this one is going to be update for 8.5 and Tile?