Try restarting the iPad (hold down the Sleep/Wake button to power off, then power it back on) and then trying the download again. If problems persist, go here:
and follow the instructions to report the issue to the iTunes Store.
Perform a Reset on the iPad (different from just restarting).
Hold down on both the Sleep and Home buttons at the same time for about 10-15 seconds until the Apple Logo appears. When you see the Apple logo on the black screen, let go of the buttons.
After the reset, go to "Settings" then "iBooks". Go to "online content" and turn it on. Try downloading from iBooks again.
Hope this answers your question!
Did happen to be the FREE iBook "Rush: Inside Racing's Greatest Rivalry"? Same thing happened to me, except my iBook is now frozen on this gray screen. I've tried resetting and powering off both and nothing has helped. I emailed support and will post what they tell me.