In practice, I've found the easiest way to work in this area is with a combination of things already mentioned or eluded to:
- widget demos are a handy start
- Mastering Tk adds things not obvious in the demos
- tools that generate code fill in most of the simple areas (vtk, specperl, glade, ZooZ). You'd be silly to rely on them completely, but they can give a start.
There's also the tutorial
here with pointers to stuff...