I'm amazed that no-one has mentioned Neil Van Niekirk!
Reading his book changed my (photography) life when it comes to flash, at least. Google his 'black foamie thing' (yes, that is its real name) - I used one of these at my cousin's wedding and it was brilliant. I used it for this shot below, balancing ambient and flash. I have a Stofen I’ve used once and hated - it’s not very selective, it just brightens EVERYTHING so you lose any kind of depth or shading. I bought Neil’s book – it tells you how to meter with flash, bounce, diffuse and balance – but all the info is on his huge website if you’ve time to scour it from the beginning.
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ETA: Oops, I see edge100 has, in fact, linked to Neil!