At the time we were led to believe it to be the first of a duology, but it turns out McDonald decided to write a trilogy after all.
I’m not sure that decision paid off. It’ll probably pay off in terms of total copies sold, but artistically, Luna: Wolf Moon has some problems. Not that you shouldn’t read it. If you liked New Moon, you’ll enjoy Wolf Moon too. But while overall solid & fun, it simply isn’t an as good a book. Not even close.
Writing trilogies is tricky, especially if they are one story, multiple volumes. I feel, ideally, every volume should have its own voice, or, at least, should bring something new to the table. Otherwise, the decision to go for multiple volumes published some months or years apart is only motivated by practicalities and commercial interests, not by artistic considerations.