[kjk]

http://db.tidbits.com/article/11404

On the Kindle, or in any of the Kindle apps, you register a particular Amazon account, and once registered, any books you've purchased appear (they appear in the Archived Items view, so you must access them to move them to your Home view). Nothing prevents you from deregistering your account, and registering, for instance, your spouse's account. Once you do that, your spouse's books appear in the Archived Items view for you to read, in addition to your existing books.

It turns out that Apple implemented nearly the same system with iBooks and the iBookstore. From the main Library view in iBooks, tap the Store button, and then tap Purchases in the toolbar. The Account button shows you the current iTunes account that's signed in; tap it to bring up a dialog that lets you view the account and sign out. Tap Sign Out, and in the dialog that appears next, tap Sign In and then Use Existing Account. Provide the other account's username and password to sign in.

Seems easy enough.