I would be concerned too if the books I purchased from iTunes were not downloading. I have a few suggestions to get it downloaded to your iPad.
First, I recommend checking to see if the download was interrupted:
If your download was interrupted using your iPhone, iPad, or iPod touch
- From the Home screen, tap the iTunes app.
- For iPhone or iPod touch, tap More > Downloads. For iPad, tap Downloads.
- Enter your account name and password if prompted.
- Tap the blue download arrow to resume.
If the book still do not download, next I recommend redownloading the book from your list of past purchases. Follow the steps in the section titled "iPhone, iPad, or iPod touch > Books" in the article below to download the book:
If you are still not able to access the books, you can report an issue with these purchases using the steps in this article: