Grand Canyon Association offers the following ebooks:Carving Grand Canyon: 2nd Edition
I Am Grand Canyon
Bright Angel Trail Guide
South Kaibab Trail Guide
Grandview Trail Guide
You can purchase epub files here: http://mytabletbooks.com/Index.aspx?merid=406 You will be taken away from the this site.
Grand Canyon Association ebooks are also available on Amazon, iTunes and Barnes and Noble.