Dent, B. D., 1999. Cartography - Thematic Map Design 1st Edition. WCB McGraw-Hill. [electonically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey ] Chapter "Techniques of quantitative thematic mapping" especially
Lewis, P., 1977. Maps and Statistics. Methuen & Co Ltd.
Slocum, T. A., 1999. Thematic cartography and Visualization. Prentice Hall Inc.. Chapter "Data Classification" especially