Did Jake save Cassie or the world? I know it was intentionally left open, but what do you think? I'm leaning towards Cassie, but most of my clues can go either way.
He leaped for "what must be valued above all else". That can be taken to mean loved ones or freedom.
He called Cassie immediately afterward. Either to reaffirm her safety or to confirm that it was all a dream (and she didn't actually die).
When he says "I knew what I was made of. My limitations and my priorities." I take that to mean Cassie, as he shows comfort that he could draw the line somewhere, ending his debate over leaving Marco and Rachel to begin with, and if he'd chosen the world it'd mean that he had no limits. But at the same time, it could mean priorities on saving the world and the limits for which he'd go to save his friends.
Then there's Justice, the kid he identified with. On one hand, Justice believes in saving the ones he loved first, and others second. On the other hand, Justice would probably die if Jake saved Cassie, and who sends a kid to death?
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Also, what are your thoughts/speculations on the entity at the end of the book? Personally, I think it was all in Jake's head, a dream of an alternate future, which would suggest the alien had something to do with the mind (or thoughts).