@SmilerLoki, I couldn't convert you, could I? 
Alas, not this time, though I still quite liked Book 3.
I enjoyed Tavi as a character, and I was invested in his story/development, etc... I thought the ending came together well enough, though a little out there for a few chapters.
He kinda felt grossly lucky, his inventions were all obvious (not only obvious, but literally requiring people to forget what they already know to not use such implements for hundreds of years), making the world around him full of idiots, and I feel that the idea of always coming up with workable third option just plain lies to us. For that matter, Sextus was very realistic: sometimes you are forced to make ugly decisions. That's what you should be ready for, not some fairy tale where you think hard enough, make an effort, and everything magically works out.
But then all characters seemed really one-dimensional. If someone's a monster, they're just a monster. If someone's a spoiled brat, they're always a spoiled brat. If someone's a good man, they'll be good no matter what. Tavi is clever and wants everything done neatly, and that's his entire personality. Even a bit of conflict he has because of his abilities looks very artificial, since he wriggles out of impossible situations time and time again regardless. Everyone else pretty much lacks even this bit, or it's kinda maybe there, but not really, because it has absolutely no consequences and is resolved instantly
(think Isana's fears and her epiphany in Book 4)
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There is a lot of things like that.