I don’t remember how I originally stumbled across this, but looking through my ‘Embedded’ bookmarks I was reminded of this interesting and relatively accessible introduction to basic sensors.  Rather than focusing on sensors, though (temperature, humidity, etc.), it focuses on something much more interesting … the ways that many sensors actually measure things (via resistance, capacitance, inductance, etc.).  If you’re looking for a basic primer of the advantage and disadvantages of different ways of measuring the physical world around us (visible or not), this is as good a starting point as any!  Head over to Introduction to Sensors by the University of Exeter for more information.