The books you purchase from us are wrapped in a zip file that the native software on the iPad doesn’t know how to unpack. The sample books are plain PDF files so those work as-is. You have a few options:
1. You could download the file to a computer, unpack it, and then drag the pdf file into iTunes, and sync it to your iPad.
2. You could download the file to a computer, unpack it, and e-mail it to yourself on the iPad.
3. You could install an app on your iPad that can deal with zip files and many other formats. I really like GoodReader. It can read just about any kind of document. You can view PDFs from within GoodReader or send it to iBooks once the file is unpacked. GoodReader gives you a lot of great ways to get content into your iPad, plus you can add notes and annotations to the pages.
I have not tried other apps because I like GoodReader so much, but one of these might be a free alternative: iUnArchive Lite, CloudReaders, FileApp, iFileExplorer.
Be sure to let us all know if you find an app you particularly like.