@herself Funny You Should Ask (a facts book); or, you may find something at standardebooks.org, project Gutenberg, and the like (although that’s not so much a recommendation as a place of many books)
@cn Funny You Should Ask sounds perfect! Might have to look into that for a birthday present.
In the end I had to get him a gift card (boo) because I couldn’t be sure if his kindle was registered to Amazon.com or .com.au (and you can only redeem a gift of an actual book at .com)... he seemed happy though.
@herself I was given a gift voucher for Amazon.com a few weeks back and have finally bought enough ebooks to use it up. Didn't have to buy a physical book at all…