This book provides good, general information about the Porsche line of cars from the first 356 up to the '97 Boxster. Information is generally more detailed about the earlier cars (pre '80) and then the author(s) seem to run out of steam, adding only brief paragraphs to describe modifications during the more recent years. This is perhaps understandable, as all Porsche cars are evolutionary, with small changes being made over the years.
The book will be useful if you are just starting to think about buying a used Porsche, and aren't even sure which model interests you. If you have already narrowed your search to a particular model, then there are other sources of information. For example, there is a great webpage of the evolution of the 911 which contains most of the information contained in this book on model changes. The Early 911 FAQ website also has a lot of detail on individual model year changes, plus some performance information and a great section on the possible trouble spots.
This book will not provide enough information for the final selection of a good used Porsche. It warns of the most obvious troublesome Porsches (ie. 2.7L 911s) but lacks the details for some other common problems that are smaller, but potentially very expensive if overlooked. Consider the book a good primer, but definitely look elsewhere for detailed pre-purchase advice.
