This edition includes a number of minor changes. First, I have corrected all the errors found so far and, hopefully, I have not made new ones in the process. Thanks are due to many people who, mostly using the wonders of the email age, have sent their encouraging words as well as their suggested corrections. I have attempted some improvements to the presentation of certain concepts; questions posed by readers have been helpful in this respect. Having used the book for lectures, since the publication of the first edition in 1997, has been useful in re assessing some of the material and its mode of presentation.