The Realms of the Gods by Tamora Pierce

Daine and Numair are suddenly swept into the otherworldly home of the gods after facing certain death on earth. But they cannot remain there for long, because they are both needed to help fight the desparate battle raging in Tortall. And so they undertake the dangerous journey back to earth...a journey that will teach them a great deal about life and about each other, a journey that will lead to the startling culmination of the conflicts, both mortal and immortal, that have long plagued Tortall. (Fourth in the Immortals series)

This was my favorite book in the series when I was younger, but not so much now. Yes, it is partly due to the controversial romance. But it's not exactly the age gap I object to—this is a fantasy novel, I don't expect our morals to map 100% onto the fictional world of Tortall—but how it's handled. Pierce makes a point of presenting the moral dilemma in Daine and Numair's relationship when Numair says the age gap, the power differential, and the fact that he's known her since she was 13(!) makes him uncomfortable....but instead of addressing that at all, the issue is waved away by Daine deciding that's she's totally mature enough for a relationship with her mentor, actually. Which honestly just makes her seem more childish! There also isn't a transition between Daine-the-kid and Daine-the-lover, either for the characters or in the narrative, so that whole storyline comes across as tone-deaf and inadvertently creepy. Just not great writing IMO.