This would have been 5 stars, except I'm getting a bit tired of Ms. Lackey's overuse of the "Bad guy schemes to take over someone else's body" plotline. Off the top of my head, I can think of three other books, one in this very series, that she's used it in, and I wish she would find some other type of villainy for her villains.
That caveat aside, Ms. Lackey does a good job with the characters and the setting. Clearly, she has done some research into the horrors of World War I, and they are interwoven so skillfully that none of the information comes across as a history lecture. Miracle of miracles, Ms. Lackey even made me care about a character based on Dorothy Sayers's Lord Peter Whimsey, who I cannot stand.
Overall, this is an engaging summer novel, with enough action to make you pay attention, characters to enjoy, and enough bits that make you think without boring you to death.