This classic work gives an excellent overview of the subject, with an emphasis on clarity, explanation, and motivation.

Lee A. Segel (1932-2005) was the Henry and Bertha Benson Professor of Mathematics at the Weizmann Institute of Science. He also served as Head of the Department of Applied Mathematics, Dean of the Faculty of Mathematical Sciences, and Chairman of the Scientific Council. Professor Segel taught at institutions throughout the United States, most recently at the Santa Fe Institute.