In recent months, Desiring God has published a number of smaller works in ebook formats (PDF, ePub, and Mobi). The latest of these is Disability and the Sovereign Goodness of God by John Piper and Tony Reinke.
The book is about 70 pages long and looks to be very helpful.
Download the free book, Disability and the Sovereign Goodness of God at Desiring God.
Many of the other books by John Piper are also available for free in ebook formats on the DG website (many of them are currently available only as ebooks in PDF style). See all the books by John Piper here (not all are available for free downloads, though most are).