Just so everyone knows, when you update to iOS4, you will have to re-download your offline books. I'm going to be a while...
I just updated to ios4 and did not lose my offline books. Did you log out before upgrading?
No, I stayed logged in, so I don't know what happened to my books. Maybe someone can give some pointers than for updating iOS4 so others won't have to redownload all of their books.
I just did a regular update of the OS to iOS4 and my off-line books are OK.
