In the past, there are web pages and an old porting guide for itcl on the [1] web site. Also, a lot of the same information from the web site can be found at [1] . There is a mailing list maintained at this site.
BOOK: incr Tcl from the Ground Up. Author: [Chad Smith] This appeared in January, 2000.
A complete reference manual for itcl, as well as covering OO design issues, etc. Covers fundamentals, as well as advanced topics such as overloading, code reuse, multiple inheritance, abstract base classes, performance issues. Also covers itk and the 56 iwidgets. This is a tutorial approach rather than an encyclopedic approach to covering the material.