I am having the same problem. My payment information is current & confirmed valid (I even deleted my info and put it back in & iTunes did the standard $1 charge to confirm the info). I've turned off my phone & restarted it several times, verified my software is current & up-to-date, I've completely shut down iBooks and I've tried downloading both connected and not connected to wifi. Nothing works. It only seems to happen on some books though. Some download just fine, but other books will not download. I can get the sample just fine, but when I click on the Get tab, it very briefly shows the download circle but then goes right back to the Get tab and then the sample is erased from my library. It's so frustrating as I usually read 1-2 books a day. I don't usually pay for books, preferring to be cheap and only read the free books, so I don't know if it's an issue on the paid side too. It seems to have started in the last couple of days.
Sorry that I have no useful/helpful info, but you guys are definitely not alone with this issue.