I have that issue too. I added like over 100 books now in my library and some go away. The other day I had a weird one. I had some Kathleen Kirkwood titles in my library and they were through Smashwords. Then I noticed no cover anymore and when I clicked, it went nowhere. So I searched for the author and found them listed just fine, but under Draftsomething now. So I added those and deleted the smashwords ones. Its like she had them first through Smash and then switched to Draftsomething.
I know both services offer to go to Scribd and I wonder if that author hit the button on both sites and because Scribd saw duplicates, deleted the older ones automatically. Who knows.
Its wierd sometimes how I can't find a book by title, but find it by author, sometimes I find it, then not again and later its back.
I browse Sribd by romance-most recent often to see what was added and within a day stuff that was there as recent is suddenly gone again. Poof.
There seems to be some fluctuation going on. At least the gone books are still as place holders in the library so I can search for them again if I need to later.