Tutorials: Image Histograms
http://www.cambridgeincolour.com/tutorials/histograms1.htm
Luminance & color histograms
http://www.cambridgeincolour.com/tutorials/histograms2.htm
Understanding Histograms
http://www.luminous-landscape.com/tutorials/understanding-series/understanding-histograms.shtml
Maximizing Signal/Noise Ratio in Digital Photography)
http://www.luminous-landscape.com/tutorials/expose-right.shtml
A Practical Guide to Interpreting RGB Histograms http://www.sphoto.com/techinfo/histograms/histograms.htm
Understanding Your Digital Camera's Histogram http://www.cleanimages.com/Article-UnderstandingYourDigitalCamerasHistogram.asp