HT4623: Update your iPhone, iPad, or iPod touchLearn about Update your iPhone, iPad, or iPod touch
Currently Being ModeratedSep 27, 2013 1:01 AM (in response to rikaeng)
As long as they are still in the store then you should be able to redownload them via the Purchased tab in the ibookstore in the iBooks app
Currently Being ModeratedOct 1, 2013 3:26 PM (in response to rikaeng)
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!