Tcl/Tk libraries and papers, available at http://www.scriptics.com/ for the Sun Tcl/Tk pages; http://www.neosoft.com for everything related to Tcl/Tk.
All widgets are extensively documented. Look for the file index.html.
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.