An overview of the TCL (Tool Command Language) is in the Tcl.n manual page. You may also want to look at the products tk, blt, expect, and tktools, which are based on TCL, and provide interpreters with added commands for windowing, controlling other programs, etc.
Some off-site information, including papers, etc. about Tcl is at Scriptics.
Manual pages for individual TCL commands are:
append.n array.n break.n case.n catch.n cd.n close.n concat.n continue.n eof.n error.n eval.n exec.n exit.n expr.n file.n flush.n for.n foreach.n format.n gets.n glob.n global.n history.n if.n incr.n info.n join.n lappend.n library.n license.terms lindex.n linsert.n list.n llength.n lrange.n lreplace.n lsearch.n lsort.n man.macros open.n pid.n proc.n puts.n pwd.n read.n regexp.n regsub.n rename.n return.n scan.n seek.n set.n source.n split.n string.n subst.n switch.n tclsh.1 tclvars.n tell.n time.n trace.n unknown.n unset.n uplevel.n upvar.n while.n
Manual pages for library routines in the tcl library are:
AddErrInfo.3 AllowExc.3 AppInit.3 Async.3 Backslash.3 CallDel.3 CmdCmplt.3 Concat.3 CrtCommand.3 CrtInterp.3 CrtMathFnc.3 CrtPipelin.3 CrtTrace.3 DString.3 DetachPids.3 EnterFile.3 Eval.3 ExprLong.3 GetInt.3 Hash.3 Interp.3 LinkVar.3 PrintDbl.3 RecordEval.3 RegExp.3 SetRecLmt.3 SetResult.3 SetVar.3 SplitList.3 StrMatch.3 Tcl_Main.3 TildeSubst.3 TraceVar.3 UpVar.3