Start with this link which is Cisco tutorial
http://www.cisco.com/en/US/docs/ios/12_3t/12_3t2/feature/guide/gt_tcl.html
There are some good general books on TCL (not Cisco specific)
1) Practical Programming in TCL/TK
2) Learning Expect - another TCL tool but the first 4 or 5 chapters give a good intro to TCL.
Also if you google on the web there are many tcl tutorails available.
Jon