Extreme NXT: Extending the LEGO MIDSTORMS NXT to the Next Level is for intermediate-level users of NXT who would like to advance their capabilities by learning some of the basics of electronics. Plenty of examples are provided, and easy-to-follow instructions are included for building over 15 different sensors.
This makes a great reference for the NXT hardware interfaces. Examples even come complete with multiple, alternative NXT languages. The book is also abundant with illustrations.