Here is the link to buy The Sixth Discipline if you have a Kindle or a Kindle app.
Here is the link if you have a Nook or a Nook app.
If you need the apps, you can download Amazon Kindle app here.
Or download Barnes and Noble Nook apps here.
If you have a Kobo or a Sony Reader, Smashwords is a viable option. If not, I recommend using either Kindle or Nook apps, as they are easier to use.