There are a lot of videos on places like Youtube and Vimeo showing how to use off-camera flash, but some are definitely better than others. Gavin Hoey’s Fearless Flash Photography with Speedlights series is one of the best I’ve come across.
Books on Flash Photography
Reviews and recommendations on books related to using flash, portable lighting, off-camera flash, and strobes on location.
Scott Kelby is out with a new book that serves as a concise introduction to getting comfortable with using a flash.