Reddit mentions of Childhood's End

She probably has all of the books you've mentioned if she really likes them, bookish people usually do... A special edition might be an idea, but I won't be able to help you with that, I go for cheap paperbacks due to money ;)

I'm tempted to recommend "We" by Yevgeny Zamyatin, it's not sci-fi classic per se, but it's a dystopian classic, she would probably like it if she likes Brave New World, but again, she might already have it. Still I'm sure she'd be thrilled to get a thoughtful thank you gift from you, even if she's read it before or even has a copy... Here's a link to that book on amazon, if you want to have a look: amazon link

If she does like classic sci-fi, here are some old-school, hard sci-fi (but it's not all just spaceships and aliens) that she might enjoy and possibly even not have, since a couple of the authors aren't from English speaking countries:

• Stanislaw Lem "Solaris" amazon link
  
• Arkady and Boris Strugatsky "Roadside Picnic" amazon link
  
• Artur C. Clarke "Childhood's End" amazon link
  
• Walter M. Miller Jr. "A Canticle for Leibowitz" amazon link