Books:

a. A.K.Jain, Fundamentals of Digital Image Processing, Prentice-Hall, 1989.

b. Rafael C. Gonzalez, Digital Image Processing, Second Ed., Pearson Education,2004.

c. Bernd Jahne , Digital Image Processing, 5th revised and extended edition,2002.

d. Tinku Acharya, Image Processing Principles and Applications,2005.

e. Dwayne PhillipsImage ,Processing in C,Second Edition, 2000.

Online Refrences:

a. http://www.marginalsoftware.com/HowtoScan/image_histograms.htm

b. http://homepages.inf.ed.ac.uk/rbf/HIPR2/histgram.htm