Amazon has it being released in Hardcover on September 1st (list $39.95, amz price $35.95)...
http://www.amazon.com/Pioneer-Girl-T.../dp/0984504176
No ebook so far, but the publisher (South Dakota State Historical Society) does do ebooks of other books they've published so there may be one at some point.
SD Historical Soc press release...
http://www.sdshspress.com/index.php?&id=280&action=921