I happen to like "Chaos" by James Gleick, though she may already have a copy of that. I read the book, gave my copy to a really smart young lady who attended our astronomy club, later bought another copy for myself, re-read it, and ended up giving that copy away to yet another kid. It's well-written and entertaining.
My first choice would be An imaginary tale by Paul Nahin. Really interesting history there. Most of us secondary school teachers don't realize that [itex]i[/itex] became accepted in its role solving cubic equations, not quadratic ones. Even though that's not how we teach it today.
Alternatively, consider anything by Simon Singh. Such as The Code Book or Fermat's Enigma.
Finally, although this is probably too expensive a gift, Divine Proportions by Wildberger. In a perfect world, all high school teachers would have this book.