If you put books 1, 2, and 3 together, you have what feels like a complete story.
This one, unfortunately, has more plot holes than the other ones do, which is unfortunate because we finally get to the climax of the story. (Still no climax for our virgin prude Prince and his sorceress bride to be, though. WTF, Dain?!?)
Slightly more frustrating is that basically the whole book is a giant string of events that, had they done absolutely nothing, would have resulted in basically the same outcome.
The heroes finally get the chance to talk to the Wizard of Oz - oops, sorry, the Mothers of the sorceresses - who, unsurprisingly, have inadequate cosmic powers left to do anything to help. Oh and by the way, no single solitary person has ever breached their inner sanctum in a score of years, except now the ninja sorcerer slaves can get about 60 through. (Of course, no problem. Sigh.) And what do those guys do but steal the *children* from the village instead of just kidnapping the MMC. (Keep falling in this hole, sorry.) Why? Because Empress Mom knows he'd come after them, because MMC is just such a good man... And because the ninja sorcerer slaves need more recruits to be ninja sorcerer slaves, so let's sacrifice some of the ones we have so we can take a decade or so to train them? Long term battle strategy, I... guess?
So, off they go with My Little Kelpie into the magical woods to save the children. Bye Bye, Boys! Have fun storming the monastery! Think it'll work? It'll take a miracle... Or a legal loophole in the demon contract that let any old Istovari take over for the unbreakable curse... You know, the one for which there was no foreseeable way to break it except death? Demon ex machina to the rescue! Sigh.
Am I wishing I were gushing over this more? Absolutely. This series is enjoyable. I've particularly enjoyed Thadred's character evolution. But the MMC is still too boyish for me to take him seriously, and I found myself rooting for the demon every time he mocked him, because he kind of deserved it.
On to part 4! Here's hoping!