I think Neil van Niekerk’s Flash Photography Techniques is one of the best resources available. Joe McNally is also worth checking out, particularly for Nikon users although both Joe’s and Neil’s articles and writings are generally universally applicable across different brands. Last but not least, there’s Neil Turner’s excellent dg28 site.
What have you found yourself already? I only ask because there’s probably no point in people making suggestions that you’ve already found yourself.