ACCA course (2.3 ?)book Managing People is excellent on the subject. Also read John Ball's essay on motivation published in ACCA journal (I think there's only one issue published in 2007 so far, and this essay was in that issue).

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Are we human because we look at the stars, or do we look at the stars because we are human? Pointless really. Do the stars look back at us? Now that is a question!