Nothing in life is perfect, and after four months many readers have been kind enough to point out a number of typos and errors. These are available in either gzipped postscript or pdf format. Many thanks to the following for their careful reading of the book: . ... Greg Grunberg . ... Anthony Garratt . Alison Brennan . Manuel Berondo . ... Further thanks to Greg and Anthony for their suggestions of improvements to the text. ...