Currently, the book can be bought at Amazon.co.uk, BarnesandNoble.com and other online book retailers.
http://www.amazon.co.uk/s/ref=nb_ss_w_h_/203-4527138-0755914?url=search-alias=aps&field-keywords=pen+and+paper+diet

 
The Pen and Paper Diet: Metric Edition
ISBN 978-1-4357-1904-0

The Pen and Paper Diet: Expanded Metric Edition
ISBN 978-1-4357-1913-2

The Pen and Paper Diet: US English Edition
ISBN 978-1-4357-1812-8

The Pen and Paper Diet: Expanded US English Edition
ISBN 978-1-4357-1813-5

La Dieta de la Pluma y el Papel (Spanish translation of the US English Edition)
ISBN 978-1-4357-1938-5