Since you don't mention the title or iSDN of the book, recheck the product page for the book, click on the little "?" and see if the book is supported on the iPhone, if it is not, well, it is not supported. If it is supported, call 1.800.THE.BOOK and have CS take care of that for you.
I once had a book that would not work if my iOS Nook app was set to night mode (black screen with white text). I worked fine in day or gray setting. I don't remember which book it was, but it took me a while to figure it out. That is the only thing I can think of. You might try archiving it and redownloading it too see if maybe the file was corrupted.