this is a good start:
http://wiki.tcl.tk/11483
Consider getting _Tcl and the Tk Toolkit, 2nd Ed._, by Ousterhout and
Jones.
My absolute favorite is "Practical Programming in TCL and TK", Fourth
Edition
"http://www.amazon.com/Practical-Programming-Tcl-Tk-4th/dp/0130385603/
ref=sr_1_2?ie=UTF8&s=books&qid=1261360059&sr=8-2"
It doesn't cover versions 8.5 and up though.
TCL and TK toolkit, 2nd edition, just came out in September 2009. It
should cover version 8.5 but I haven't read it and it probably misses
the latest features. Not too many reviews on it yet on amazon.com .
amazon.com combines the reviews of the 1st and 2nd editions, but most
of them are for the 1st one.
"http://www.amazon.com/Tcl-Toolkit-2nd-John-Ousterhout/dp/032133633X/
ref=sr_1_1?ie=UTF8&s=books&qid=1261360059&sr=8-1"
Exploring Expect, this one has started me with TCL. It has an easy
read and light introduction to TCL. You will also learn Expect, which
is a full language by itself. The book is pretty old though. Published
in 1994.
"http://www.amazon.com/Exploring-Expect-Nutshell-Handbooks-Libes/dp/
1565920902/ref=sr_1_10?ie=UTF8&s=books&qid=1261360059&sr=8-10"
It does cover 8.5.
I'm aware that there's also discussion ongoing to create at least one
book to cover 8.6, but no releases (or even hard news) yet.
Donal.