If you are looking for information on creating 200bhp monsters from the air-cooled engine or detailed info on forced induction or fuel injection then this book is not for you. The book was written in the early 70s and clearly has not been updated since (prices are quoted for a lot of the items - eg $100 for 48IDAs!), however.... If you are looking to build an old school motor or a single port Vintage Speed engine then its perfect. Bill Fisher has clearly put a lot of research into the book and this shows, especially in the chapters on head work, which is extremely well done.
In summary, there are probably a lot more modern and therefore more concise books on getting the most from the VW aircooled engine, but as a resource for "Period" speed this is fantastic, and there is a good reason why this book has stayed in print for over 35 years.