How do I gift a book from the iBookstore? 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 ....

That option, however, is not available on books in the iBookstore:



How can I give someone a book from the iBookstore?
Updated 17 Dec 2013.
As reported by Macworld, the iBookstore now allows you to gift books:

Gifting in the iBookstore works identically to Apple’s iTunes and App Store counterparts: Just choose the book you’d like to gift, then tap either the dropdown arrow next to the price (on a Mac) or the Share button (on an iOS device).

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:

Can I gift a book?
No, you cannot gift a book in the iBookstore at this time.

The phrase “at this time” suggests that it’s coming in the future, but Apple haven’t announced anything yet.