This entry in the series is the most serious in tone so far, with a depressed and guilt ridden Valkyrie trying to come to terms with the events of the previous book. She's 24 at this point and inevitably we get introduced to a younger protagonist (14) because that's the Law or something; kids' books must have at least one protagonist under 20. The newbie is the Chosen One's brother. This seems like a very deliberate counter-casting to Valkyrie and a bazillion other destined heroes, with a view to saying that "ordinary" people (i.e. the young readers) can be heroes, too, or at least acheive their dreams. Additionally, we have Never, a non-binary kid referred to by "he" or "she" apparently at the author's whim, who looks like at least being a side-kick, if not a true protagonist.
People looking for the continuous quipathon of the early books will be disappointed but this is a good set-up for a new and more sophisticated long term adventure in the world of the Skeleton Detective.