Tcl/Tk libraries and papers, available at
for the Sun Tcl/Tk pages;
for everything related to Tcl/Tk.
All widgets are extensively documented. Look for the file
About Face: The Principles of User-Interface Design.
Alan Cooper. IDG Books Worldwide, 1995.
Forceful, opinionated writing on how GUIs should be designed.
Bringing Design to Software
. Terry Winograd. Addison-Wesley, 1996.
Has specialists from different fields talking about good design, especially user interfaces.