I haven't personally purchased any books on TCL for a reference, but relied on the online docs and TCL client distributions. Here are some links of interest.
1. The official TCL SourceForge Documentation -
Click here
2. ActiveTCL - Windows TCL client -
Click here
If anyone else has recommendations on TCL primers, I'd be interested in hearing about what you find useful.
Oh, and when reviewing the online TCL docs, keep in mind that we have disabled several of the builtin TCL commands in iRules for optimization and performance reasons. The list is available here:
Click here
-Joe
Nimbostratus