I have found the TkDocs tutorial to be very useful. It describes building Tk interfaces using Python and Tkinter and ttk and makes notes about differences between Python 2 and 3. It also has examples in Perl, Ruby and Tcl, since the goal is to teach Tk itself, not the bindings for a particular language.
I haven’t gone through the whole thing from start to finish, rather have only used a number of topics as examples for things I was stuck on, but it is very instructional and comfortably written. Today reading the intro and first few sections makes me think I will start working through the rest of it.
Finally, it’s current and the site has a very nice look. He also has a bunch of other pages which are worth checking out (Widgets, Resources, Blog). This guy’s doing a lot to not only teach Tk, but also to improve people’s understanding that it’s not the ugly beast that it once was.