Well, I finished the book last night.
Overall, I'd rate it a solid 9.5/10. Very good. Review follows in spoiler protection.
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Mercy kill started out more or less right into the action with a mission from the old days, when most of the original team was still together. It really showed who the Wraiths were. Either introducing new readers to them, or refreshing the memories of old readers). This seems unrelated until near the end where the rescued character becomes essential to the mission being conducted. A well-orchestrated bit of forshadowing. Not to mention that the mission itself was perfectly written.
The other two major flashbacks that occurs later when Piggy dozes off really illustrate how the wraiths changed over time. And the last flashback shows what led Piggy to where he currently is in life. It also is an excellent throwback to the feel of the NJO, and illustrates the horror and loss of that war, while only actually using 4 named characters.
Now to the modern Wraiths. How the teams were assembled (as explained by Face near the end), made perfect sense, and allowed a good mix of the old Veterans, and new characters (or ones who have had little page time), each of which had a unique personality, but all of whom could be linked in one way or another back to who the Wraiths used to be. excellently done.
The mission itself fit well with a Wraith mission, and I have few complaints on that point. Without describing it in detail, its the classic mix of brilliant tactical planning, unpredictable massive screw-ups, and a variety of humerous, unpredictable hijinx.
Overall, its hard to find much in the book to complain about. Really there's only four things that bothered me. Two minor and Two major.
a) 3 spelling mistakes that I noticed that escaped editing. (and not just a missing letter, but the wrong word, or a nonsense word entirely)
b) Wedge's 2 page appearance - if you're putting someone on a 10 character DP, he should really have more page time. especially given that several other major characters (ie, half the wraiths) weren't even mentioned there. Its also a loss for possible humour IMHO. If he was just a nice nod for a brief period, he shouldn't have gotten a DP slot.
c) Almost none of the squadron are actually pilots. Kind of gets away from the original point of the squadron. Hopefully in future books they'll be trained for that, since the purpose was an intelligence squadron that could fly in addition.
d) The ending. Not to say that large parts of it weren't done well, but not officially reinstating Wraith Squadron was illogical, especially now that Face is in charge of intelligence.