According to a blog post on Kobo.com: “As part of iOS 5, the system automatically purges data from your device when it runs out of disk space.
“Unfortunately, when this happens, your books will be deleted from your library without your permission – meaning you will have to re-download your books again.
“To avoid your books being deleted, we recommend that you update to the latest version of the Kobo app (version 5.1) AND upgrade to iOS 5.0.1 as we’ve included a fix for this.”
The latest version of Kobo’s app “includes an auto-alert to warn you that your device is near full capacity as well as a notification to let you know that the system has removed your books.”
Upgrade instructions are available on Kobo’s website.