Sensor Wiki - Open sources description of how sensors work and their applications. Sensorwiki was started in 2004 by Prof. Marcelo M. Wanderley and several graduate students at the Input Devices and Music Interaction Laboratory, at McGill University
Honeywell Sensing - Wide range of sensors including pressure, magnetic, and optical
Handbook of Modern Sensors - Handbook of Modern Sensors: Physics, Designs, and Applications by Jacob Fraden, 2010. The link is to an electronic version, but you must be on UCSd campus to access it.