Hi rah.e,
I think I can help explain the process. Corsicansailor mentions the book and preview. The "book" is your full textbook. The "Preview" is the sample version you create as part of your package that is a smaller version of the full book.
When you hit the "Publish" button in iBooks Author it creates an iTunes Producer File or "ITP" it shows up as your isbn # with the extension .itmsp in your finder after you fill in all of the metadata for your publish. You'll do this for both your full book and the smaller sample or preview book. You'll need to remember where you save it when you hit the publish button in iBooks (It's probably easiest to save them to your desktop).
Now that you have the definitions you're ready to give it a try.
1. Go into iBooks Author and open your Full Book version. Hit Publish. Fill in all of the required fields. When you get to the "Assets" tab you'll see a space for your "Publication Preview." Just leave that empty for now, but fill everything else. Leave that iTunes producer window open.
2. Now go back into iBooks Author and open up your preview version. Hit publish (pay attention to where this file is saved). Then close the iTunes producer window for your preview.
3. This is where you need to remember where you saved the preview .itmsp file. If it's on your desktop, right click it and select "Show Package Contents." There you will find a file with a .ibooks extention. This is the file you want to drag into that empty space for "Publication Preview" in your full book iTunes producer file you started in step 1.
4. Once you have all of the required metadata fields filled you're ready to deliver. You might try checking the "DRM Free" box in the "Rights and Pricing" tab to ensure delivery. You can recheck it once it's on the store.
I may have over simplified here, but either way I hope it helps you and anyone else having the same problems. We all know how frustrating this process can be.