I just did a single download to check the problem. I actually have hundreds of books on iTunes which I regularly switch in and out, with never a problem. As I said, iBooks and iTunes are right up to date, as is iOs on the iPhone and iPod. I have kept 5.0.1 on the iPad as I'm considering jailbreaking it.
The more info we can get, whether someone is having problems or not, the better, so we can try to see what causes problems and what does not.
What we are really looking for is any difference between your system and mine, I guess.
Sent from my iPad 4 HD mini