I've been thinking some about the idea of the unresolved ending in fiction, though I must say that Chestnut is mostly over it. A smart commenter suggested that series, particularly fantasy series, force you to pause in the middle of the narrative, and it's something to adapt to.
And I think this is an excellent point, but I also wanted to note that in the case of the Jo Walton book, the unresolved ending was an intentional question to the reader: which of these do you think happened? Never to be resolved explicitly by the author, in that book or any other.
Which made it clear to me why it bothered me so: Because both Chestnut and I believe that the narrative world in any given book is real.
Perhaps it's not in this "plane," and it's true that we can never enter it except through the magical conduit of a book. But—I have to face facts. I believe everything I read. In some part of me, I feel that all these fictional worlds exist, and I get tastes of them through books: Narnia exists, Wonderland exists, all of it exists, but you can only get these quick book-length glimpses. And when an author says "You decide," it seems to claim (or acknowledge, depending on your belief system) that these worlds aren't real, that what is said to happen is just based on the author's whim, not on actual fictional events in the other world.
I realize that this is maybe a little crazy? But it feels to me, in my heart of hearts (should such a place as that really exist), true.