I think this is the first book that I've really felt such an intense emotional rending between the three and four stars. So I'll give you reasons why I'd give it either.
I'll start with why I hesitate to give a definite I LOVE IT:
Not a lot of female characters in this one, which may sound like a petty grievance, but a) it isn't, and b) I've not read other Sherri L. Smith, but I was tempted to pick this up because of a lot of people telling me that she was a writer that didn't usually slack in that department, so I assume this is something out of the norm for her. The lack of female characters is heightened by where the main female human character is located in the book - intermittently and mainly at the beginning and end. The first to introduce as a love interest and to conclude with her confirmed as such. The other female human character is an object goal - and when that goal is completed and she is human again, she is immediately characterized as superficial and then her and her family more or less disappears from the book entirely.
Generally the way that the human royal family is used feels extremely superfluous to the plot - the way that they are presented in the beginning speaks nothing of how entirely absent they are for 90% of the book. In a book where life lessons seem to be pouring out of the sky, there doesn't feel like there was anything gained from the family Pirlipat other than annoyance of useless royalty. It causes the first "Book" of the ... book to feel like a 150 page prologue, nothing accomplished except the causality of the real meat of the book. I do feel conflicted on this because it's not as if the book could do without those first 150 pages, and I will defend that they're important and enjoyable, but I feel like the characters could have continued through the book and been humanized rather than being dumped unceremoniously and not really even mentioned again. A scene that stands out to me in particular is the one where Stefan does meet them and is forced through stupid, glamorous, and pointless hoops to save the princess. It's a one scene joke when it could have been stretched to more.
The grey morality of the book also isn't fully allowed to be grey. We are told that there is a war, we are told that difficult things happen in them, but the only real tragedy on the side of the humans is the death of Stefan's mother - a causality not done by the mice. The tragedy hangs over the book, and implied to affect Zacharias heavily - this was done well! And perhaps this was to emphasize the curb-stomp battle that a war of humans vs. mice would entail, but it also makes it seem as if the mice were never really a threat in the first place. I feel like the book was trying to make it feel like the mice are likely not to win, but would incur difficult casualties. So like for a hundred mice felled, a human would fall. With the streets teeming with mice, you'd expect some threat. But the book just doesn't follow through. Even in the end when the mice are flooded away, ending an entire kingdom - no question of if it was justifiable. A brief hesitation, maybe, but that's it. In a book that reveals each individual mice to be a person, it does certainly kill masses of them without a lot of deep thought. I just don't feel like there was a mutual understanding between the humans and mice. The mice pay deeply and I suppose learn their place in the world. Humans pay nothing and learn not really anything.
Why I LOVE IT AND WOULD READ AGAIN:
Its take on the story is definitely interesting and fascinating. There is an undeniable quirkiness to the book that I never felt overwhelming to the more serious aspects of the plot. The intricacy and variety of Rodentia is definitely cool, and you definitely get a feel for the shortness of their lives, where epic legends to them are a few months ago for humans. Plus, I think it was unique to have the sapience of mice being a well known thing - typically books with mice societies involve them being secreted away, living under heels and returning to unknown metropolises hidden to humans. So one where they are out in the open got my ears perked throughout the book.
I do like the grey morality that the book DID accomplish, especially in Arthur, who is really characterized beautifully - to me probably the most strongly. He was such a strong character to me that was I desperately trying to figure out a way for his story to end well; magic to split him from his brothers? battlefield surgery of the rest of his heads? Arthur's development as the level-headed (pun unintended) and emotionally lonely of the bunch was definitely felt deeply to me. His conflicts about his feelings to his mother, his destiny, to humans are complicated and not easily solved, and you can see how it culminates in his character towards the end of the book, as sad as it is. And that development is slow and measured enough to believe, and the ownership of his boundary crossed didn't make me feel like he was a character thrown away. I didn't feel like Hannibal forced him into a revenge frenzy or anything. By the end of the book I did feel that Arthur was his own mouse, and his ambition was a part of his downfall.
Generally, the characters, when rounded, were written very well. Ernst's horror and revulsion upon seeing the new Mouse King(s) - I felt it too, as was the growing nervousness of coming wars doomed to fail. Plus, Ernst, as his role in the story, was unique. I expected some sort of ambitious or overtly goodness to be about him, instead his need for survival made him a likeable character to me in that dodge of trope. And I did like Christian a lot - and the difference in appearance and emotion you feel for him depending on who you're with. Perhaps something to be more felt if Stefan could go on to acknowledge that Christian's attempts to eradicate the mice were horrible, but! As it stands the switching of perspectives was handled well and did a lot to push the "no right answer" tone I believe the author wanted.
And even if I did feel like no humans died other than Stefan's mother, the blanket of atmosphere and feeling she leaves behind in her wake is heavy and unavoidable. Partially in Stefan, mostly in Zacharias in his prison cell, feeling like his family is dropping like flies.
And I just liked the book. Overall. I liked the tone, I liked the characters, I liked the concept and story. With my grievances set aside, there is enough left to like about the book for me to enjoy it and recommend it to other people.