Great, informative article by Nasim Mansurov over at Photography Life:
Due to the fact that we as humans cannot be completely still, particularly when hand-holding an object like a camera, the movements caused by our bodies can cause camera shake and introduce blur to images. The basic premise of the reciprocal rule is that the shutter speed of your camera should be at least the reciprocal of the effective focal length of the lens. If you are confused by what this means, don’t worry – it is really easy to understand once you see it in an example.
Really useful and practical advice. I found the Auto ISO section to be particularly interesting, as I had always wondered how the optimum ISO was determined when using this feature.