This is the 4th edition of the book, and it is available as a free download in PDF format.  Distribution and copying are not restricted.  There is no authorization for editing the text or the book's content.  Unauthorized changes are a violation of copyright laws.  This book may not be sold by any person when distributed as a digital format.  Printed texts cost money, and only the cost of printing may be charged when distributed in a physical format.  Small corrections in grammar or punctuation may occur in this digital file, and updates will be posted to replace error-corrected copies.  For example, an error in the numbering of segments for Part Four has been corrected.