Neil van Niekerk has a pair of flash photography books that I read last year and thought were well suited for intermediate photographers with minimal equipment:
- On-Camera Flash Techniques for Digital Wedding and Portrait Photography
- Off-Camera Flash Techniques for Digital Photographers

You don't need to be a wedding photographer to benefit from the first book as the information is transferable to any situation for which you could use an on-camera flash. The second book primarily deals with using a single off-camera strobe, usually a speedlite, with and without light modifiers. Both books are written for both Nikon and Canon equipment.