eBook versions of all of Joseph Pilates’ books are available in all of the most popular versions (iPad, Nook, Kindle) and a host of others as well:

iPad, iPhone, and iPod Touch versions available at the Apple iBookstore
Kindle version of the Millennium edition at Amazon.com. Search there for Kindle versions of other books.
Nook version of the Millennium edition at Barnes & Noble. Search there for Nook versions of other books.
All others: Click for eBook Distribution List