In the novel's last sentence, Mrs. Wix marvels that:
She still had room for wonder at what Maisie knew. (XXXI.154)
This means that a whole lot of the book is summed up in those final words: almost the entire novel focuses on what Maisie has learned. But these words also suggest that "what Maisie knew" at the end differs from what she knew at the beginning. This girl is nothing, after all, if not a learner.
But check it out—we have two learners present in this sentence. Both Maisie and Mrs. Wix are described as keen on the learning proces—which is possibly why they're made for each other. Mrs. Wix still has "room" (in her mind) to "wonder" (and learning, at least after school is over, is always preceded by wonder) at the contents of Maisie's little noggin. These two keen learners are together at last, and Henry James, in a rare moment of optimism, suggests that they might just live fairly happily ever after.