On the iTunes Store, you can gift people music, apps, etc. by clicking on the down arrow next to the buy button and choosing Gift This ...
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That option, however, is not available on books in the iBookstore:
How can I give someone a book from the iBookstore?
Best Answer
Updated 17 Dec 2013.
As reported by Macworld, the iBookstore now allows you to gift books:
More details are in their article, but by-and-large it seems to behave exactly as you would expect.
My original answer:
You can’t; iBook gifting isn’t supported yet.
From Apple’s iBooks FAQs page:
The phrase “at this time” suggests that it’s coming in the future, but Apple haven’t announced anything yet.