I was lucky enough to find this book for a dollar. It became very quickly one of my 5 favorite math books (out of 100 or so.) I had no idea what one could do with discrete summation. The chapter on discrete calculus (where "dx" = 1) is currently making me shake my head in admiration. Generating functions are also covered and threaten to dent my cranium.<br>The math is as beautiful as it is powerful. Even the typeset and the notation is unique and impressive. The cover is perfect. That sigma in concrete perfectly symbolizes the material and style within.