I absolutely loved the first book of this series, The Just City. I didn't love book 2, The Philosopher Kings, nearly as much, but I gave it four stars based on my adoration for book 1 and the hope of a strong ending to the trilogy in book 3. I'm sorry to say that this book, Necessity, was pretty disappointing. The first book was just so meaty and fascinating - it took an idea (the gods creating a city as an experiment to see Plato's work in action) and gave it space to breathe. There was no shortage of discussion, debate, and curiosity. It was a book of the mind, and I loved every word of it. But book 2 and, even more so, book 3 veered further and further away from this initial premise. This book was so sentimental. Again, we were given a fascinating premise - first contact between the Platonic cities on their new planet and humans from the world that was - but unlike book 1, this premise was barely touched upon. We even got a second interesting premise (in case the first one wasn't enough, I guess?) - Athena lost in the primordial Chaos outside of time - (view spoiler)[but it was wrapped up in a page of poetry, and suddenly everyone was back. Don't float something amazing like "rescue mission to PRIMORDIAL CHAOS", or to a lesser extent, "Apollo writes a divine song that will explain EVERYTHING", and then have it happen off-page. Anticlimactic, to say the least!! (hide spoiler)] Hell, remove those ideas, and just give me a book that explored more of what it meant for this Bronze Age society to make a new life on an alien world! But instead, this book was basically a treasure hunt in order for characters from the previous two books to make cameos and/or rejoin the story. 
 I wanted this book to be a glorious finale, taking the promise of books 1 and 2 and seeing it through to its conclusion - creating Plato's Republic as a thought exercise is all well and good, but will it survive contact with other societies? - but mostly it felt like we're expected to believe all the hard work is done, the society is perfect, and instead we get a weird farewell tour courtesy of the gods and their time-traveling abilities. 
 It was entertaining, and I'd still recommend the series in general and book 1 in particular, but I'll do so with the recommendation not to expect the same quality as book 1 through the entire trilogy.