How do I gift 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.