Looks like "Rainbows End" to me, no apostrophe. The Kobo UK site shows "Rainbow's End" in the site title text, but not on the cover - so looks like someone made an error there. A preview of the content shows no apostrophe. ( Edited to add: seems Amazon UK and EBooks.com UK have exactly the same problem, so maybe the distributor thinks it's a UK/US spelling variation? That's a worry.
)
You know what's really funny? Amazon AU have BOTH. Apparently
"Rainbows End" by Vernor Vinge is science fiction, while
"Rainbow's End" by Vernor Vinge is literature. So now you know.
