David Hobby produced a series of DVDs called Lighting in Layers that explains how to use small battery powered speed lights to produce amazing lighting effects. The videos show subtle lighting and dramatic lighting, with and without modifiers. There is lots of discussion on how shutter speed and aperture can be used to mix ambient light and speedlites.
The videos are well worth the cost if you are interested in learning the art of off-camera flash.
David Hobby is a local(ish) photographer in my area whom I find to be extremely talented. Having read his Strobist site, subscribed to his blog and watched his “Lighting in Layers” DVD series, I learned a great deal about working with small hotshoe flashes.