iBooks repeatedly deleted from iPhone and iPad
My iBooks keep getting deleted from my iPad and iPhone. I'll pause for a time in reading a book and when I come back to it, I have to download it again. The books are available in the cloud, but I often discover that they been deleted at inopportune times when I don't have wifi and, sometimes, not even cell service. I can't identify a setting that appears to either allow or prevent books from being deleted. I'v seen several posts on the forum where folks's books have disappeared completely, but none related to the issue that I've run into. Has anybody had this problem? If so, is there a solution?
iPad (4th gen) Wi-Fi, iOS 9.1