I also have an ARC of
The Mirror and although I haven't written/posted my review yet, I totally agree - it's a middle book through and through.
But I'm not sure there is going to have been enough story line for a three-book set, because part way through this one, I was wondering if it was going to be a duology. Minor spoiler:
We do get some back-story that way, but some of it felt kind of inessential - although, of course, it could all turn out to be essential in the third book. Although I have my doubts. But it was fun meeting some minor new characters, such as Cleo's grand-mere. And I like the found family aspects of it a lot.
And I also agree that the end of
The Mirror isn't as cliff-hanger-ish as the first one, which also really annoyed me, whereas this one, I'm kind of meh about. And then the way Roberts starts
The Mirror makes what was a big cliff-hanger from the first book seem sort of pedestrian, which was also kind of annoying. Maybe I'll be surprised and the third book will be quite a firecracker, and I hope so. But for now, for me,
The Mirror was kind of a yawn. I rank on stars, and I'm going to give it 3 1/2 stars out of 5 once I get my review written.