One for the photographers amongst you

I like taking pictures n that. I think I'm quite good at it; but I want to get better, particularly in regards to use of flash and artificial lighting. Practise and experimentation aside, can anyone recommend a book or course which they've found useful to their progression? I have a good understanding of all the basics so looking for something at an intermedite level if that's not too vague...

If you want to experiment with off camera flash, equipment wise the cheapest way is to get started is to pick up the following:
A sync lead
A hotshoe flash
A half decent lightstand stand (manfrotto 092B is a good bit of kit)
And an all in one umbrella. (shoot through and reflective)
Then get experimenting.

The above can be bought for very little cash compared to proper studio flashes, and you can still achieve very professional looking results, there are also a wealth of really good light modifiers (softboxes, snoots, etc) out there available for hotshoe flashes.
Many of Joe Mcnallys images for instance, were lit exclusively with hotshoe flashes.