If you have multiple iPads in your family and want to share purchased iBooks without having to re-purchase each one, then there is a simple solution that doesn’t require you to permanently share an iTunes account.
From the iBooks app, log out of your own account and log into the account with the desired iBooks. Then, go to your purchases page, and press the re-download button next to the books you want. Now the books will be in your Library in addition to your own! Don’t worry, this method doesn’t delete the books that you already have, although ePub or PDF books will have to be synced manually.
Apple lets you do this because there is no danger of mass-sharing, and that would require you to give out your account information which is linked to your credit card, so anyone could make purchases. Next time you try to sync your iPad with iTunes, you’ll have to authorize the computer, and this only works for up to five accounts.