I enjoyed this book about as well overall as Conflict of Honors. It has some of the same faults as Conflict -- the characters feel a little flat, the male lead is SO GOOD at absolutely everything, and there's not a lot of tension for most of the story. But it has a bunch of good emotional bits, the main characters are likable, and I generally enjoyed reading their story. Also, I had been so annoyed by the lack of romantic denouement in Scout's that it felt great to finally read that part. So I'll give it a 7.
I can't discuss the genre on this book without spoilers. That said, I will strive for a minimum of spoilers.
In the romance genre, the book has to end happily and with the romantic leads united. Mouse is a romance in every respect except this one. In fact, while Conflict works as an action/adventure, Mouse doesn't work as anything but a romance. The majority of the book's tension and almost all of the narrative is focused on the character development of the two protagonists and on their relationship. It does a good job at this. If you stop reading at the end of chapter 36, it's a romance. But the book goes on from the apparent happily-ever-after at the end of chapter 36, and ruins it in the remaining 30 pages. The actual end is bittersweet at best.
I can't say the depressing events of the denouement were themselves surprising, because I read Agent of Change and knew what was coming. But I was shocked that Lee and Miller chose to include them. Happy endings are a matter of knowing when to stop, and the happy ending for this book would've stopped it about 25 pages earlier.
So there's that.
I'm still glad I read it, but I do want to warn romance readers that it's NOT a happily-ever-after romance. D: