I purchased an expensive technical monograph (> $100 (US)) from the iTunes store for use on my 1st generation iPad. It downloaded correctly and I used it for several months with no problem. A little over a year and a couple of iOS upgrades later, I tried to refer to the book and found it was not visible on my iBooks bookshelves, and not found in search either.
The book does show up in the iTunes folder on my Mac desktop machine, and it shows up in the list of books in the iPad management tab in iTunes on my desktop machine. I have iTunes configured to synchronize selected books, and the book is checked. I'm not having this problem with any other books on the iPad.
I've noticed that the book is no longer available from the iTunes store, so I wonder if the problem is a digital rights management issue, and the iPad can no longer verify that I have a right to view the book. The iTunes store customer support service is unfortunately useless, as I can't get them to actually read my email before they respond with "No refunds" or "Restore from backup".
Any suggestions for fixing or troubleshooting this?