I've had this happen as well and now it appears I'm losing my books too! I was halfway through a book, I bookmarked the last page I was one and later when I came back to read it the novel was gone but the bookmark was still there and I didn't do anything except turn the power back on. I'm not sure what happened to the novel because it was there not 1 hour before.
edit: I tried the soft reset and it appears all my stories are back. not sure what went wrong but I'm going to blame the Sony Connect Software on this one...