This my take on it, from my SFF blog. I pretty much agree with the comments so far!
I am slowly working my way through Miéville's canon, this being the fourth I have started (although only the third I have finished - see my review of The City and the City). His stories are remarkably varied and he seems able to ignore the supposed rule that no SFF book stands a chance of being published unless it is part of a series which will hook the readers and guarantee continuing demand. Having said that, his tales (the ones I've read so far, anyway) do have one thing in common: the most important feature of them is not one of the characters, but a city. And not just any ordinary city, but one which is fantastical in some way, such as the baroque fantasy settlements in Un Lun Dun and Perdido Street Station or the bizarrely divided one in The City and the City.
Embassytown is no different, the city of this name being the only human settlement on Arieka, a planet populated by enigmatic aliens who have permitted Embassytown's establishment within their own city. The Aliens, known as Hosts, are masters of biotech and everything they make and use is alive, including their buildings. Arieka's atmosphere is poisonous to humans but the Hosts have engineered living atmosphere machines called aeoli, which maintain a blanket of breathable air over Embassytown.
Dealing with the Hosts is very difficult because of their unique Language, which they are born already able to speak. They have two voices which they combine when speaking, and are unable to understand any speech which is not made in the same way. The only humans who can communicate with them are pairs of identical clones, each pair being regarded as one Ambassador, who are trained to think and speak together.
Avice Benner Cho is a native of Embassytown and one of the few who had been given permission to leave - in her case to become an Immerser, a spaceship pilot. Now she has returned, in time to witness a remarkable event; the arrival of a new and very different Ambassador who has a dramatic effect on the Hosts. The story follows Cho's attempts to understand what is going on and prevent disaster, in the face of opposition from both the human and Host populations.
The story gets off to a slow start and it took me a few days to get into, but the pace gradually accelerates and I read the last half in one sitting. The setting and the plot are both novel and intriguing, as is usually the case with Miéville. The aliens are very definitely alien in both appearance and behaviour, although I never did form a clear mental picture of exactly what they looked like. On the downside, none of the human characters made a great impression on me. Despite this, the book is well worth the read, although it has failed to displace The City and the City as, by far, my favourite by this author.