Is there a gentle introduction to learning expl3 syntax

Question

Possible Duplicate:
Programming with LaTeX3

I was going to post a question about creating a switch within a command to determine formatting for a command and came across the following link String test, with complicated arguments in which I discovered the expl3 equivalents of \makeatletter and \makeatother in \ExplSyntaxOn and \ExplSyntaxOff. So I tried to make my command as follows:

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\NewDocumentCommand{\mytestcmd}{ O{a} m}
    {
        \prg_case_str:nnn { #1 }
        {
            { a } { A }
            { b } { B }
            { c } { CC }
            { d } { DED }
        }
        :#2
    }
\ExplSyntaxOff
\begin{document}

\mytestcmd{Hello}

\mytestcmd[b]{Hello}

\mytestcmd[c]{Hello}

\mytestcmd[d]{Hello}

\end{document}

But then I got the unexpected result that my colon from :#2 was not showing up in the actual rendered text. I'm sure there's a simple solution for this. I'm not actually so interested in someone telling me how to fix is particular issue with the colon.

However, I do find the expl3 documentation quite daunting. Is there an expl3 primer or some kind of expl3 for dummies around where I can learn some thing without feeling like I'm drowning?

Incidentally, normally I would handle the switch as in the following MWE:

\documentclass{article}
\usepackage{xparse}
\makeatletter
\def\my@test@cmd@a{a}
\def\my@test@cmd@b{b}
\def\my@test@cmd@c{c}
\def\my@test@cmd@d{d}
\NewDocumentCommand{\mytestcmd}{ O{a} m}
    {%
        \def\my@test@cmd{#1}%
        \ifx\my@test@cmd\my@test@cmd@a{A}\fi%
        \ifx\my@test@cmd\my@test@cmd@b{B}\fi%
        \ifx\my@test@cmd\my@test@cmd@c{C}\fi%
        \ifx\my@test@cmd\my@test@cmd@d{D}\fi%
        :#2%
    }
\makeatother
\begin{document}

\mytestcmd{Hello}

\mytestcmd[b]{Hello}

\mytestcmd[c]{Hello}

\mytestcmd[d]{Hello}

\end{document}

What I like about the expl3 syntax is it seems to be much kinder in building a true switch than my work around.