This book spans five Aprils and covers a huge chunk of Jethro's young adolescence, so there's pretty much no way this isn't a coming-of-age tale. Still have doubts? Let's focus in on death and compare how he handles it at the beginning of the book versus at the end.
In the beginning Jethro "squirmed inwardly" at the mention of his dead siblings (1.34), but when it comes time to record Tom's death in the family Bible, Jethro voluntarily joins Jenny as she writes, and even initiates a somber conversation with his sister about their siblings who died. He's gone from super uncomfortable about death to giving it serious consideration and willingly participating in a death ritual (writing Tom's death in the Bible). It's a pretty big transformation, and a clear sign that Jethro's done some serious growing up.
And if all the war-talk doesn't hit you over the head, let us also mention that this book falls under the historical fiction and war drama genres as well. Contain your surprise. Hunt could have just set her made-up story during the Civil War and called it a day, but instead she goes all out and namedrops generals and battles all over the place—Grant, Lee, Donelson, Antietam, to name just a few. It feels like we're actually fighting in the war as we read, and it's pretty much impossible not to learn a whole lot about the Civil War in the process.