I have downloaded other books (free and paid) which have appeared to be fine.
I figured out how to share epub files so I had my friend try the faulty copy of the full, paid "Second Nature" by Alice Hoffman. And he has the same problems with it. It's like the formatting is messed up on the file. The file size seems right for a full book, but it only appears to be 18 pages long when you look at in iBooks. It looks like all of the text for each chapter is trying to fit onto the first page of the chapter. So there are 18 pages because this book has 18 chapters. The title pages are messed up too. All the text runs into each other like the line spacing and tabs are missing. Duh - I should make some screenshots. I'll do so when I have internet. No wifi at work...
I should also say that I've tried restarting the iPad, deleting and re-downloading the book, and even redownloading iBooks. I will have to try re-downloading the Walt Whitman sample - maybe it was fixed since I posted my message, which would explain why it looks fine on your iPad. My instincts say this is a problem with the epub file itself.
But my friend and I have seen some problems with the iPad that were resolved by restarting. The iPad is just like our iPhones - when you add new apps, it's best to restart it before using anything. Unfortunately, in this case it didn't fix the books.