Let's get my main impression out of the way first; this book disappointed me.
Everything was fine for the first half. Ten years after the events of the first two books, Melisande's son, now ten years old, goes missing and she asks Phèdre for helps as she cannot find him.
The plot of the book consists out of her trying to find the boy and to look for a way to free Hyacinte from his curse. And that in a sense is also the problem with the book, because as a plot it's too thin. Once the first half of the posed problem is resolved, you are effectively done. You are only halfway in the book but from that point on you can accurately predict how the remaining hundreds of pages are going to proceed. The decay I noticed in the second book continued in full blaze in this one.
To put it simply, without a mystery to pursue the entire thing falls apart. The characters simply aren't interesting enough, and near all of the (few) choices they make are all foregone conclusions. The author's prose actually becomes a hindrance here and I skimmed through most of the latter half of the book. And I could do so easily, without missing a beat, which is both a bad and telling sign.
The next trilogy in this series will switch to a different protagonist, but I'm done with these all the same.To me this trilgy was one of an author with an excellent start, but lacked the ability to maintain the quality.
That said, if you like the prose and feel the characters are fine as they are you can not go wrong in reading this last piece, nor, I expect, the novels that follow. I myself won't be reviewing them.