Zoe's Tale is a sequel to Old Man's War and The Last Colony. It is also supposed to be a stand-alone young adult novel, and it probably works ok, but I have to admit that my first reaction to reading it is an overwhelming desire to re-read the first two books.
I find reading this whole series to be fast & easy. There's nothing absurdly complicated going on with the language. Scalzi isn't playing games with sentence structure. It is really reminiscent of Heinlein (glorious adventures in space with really intelligent main characters and interesting interpersonal relationships), which may be why I find it so easy to skim through - I grew up reading Heinlein. It isn't just fluff either, there are really interesting scenarios, huge problems to solve - it is a great story.
This particular novel I really enjoyed. Zoe is a great character, and feels very much like a typical teenage girl - who just happens to be coping with the weight of an entire sentient race on her shoulders (she is goddess/mascot to the Obin - they're observing her in order to learn how to use their newly acquired consciousness). How she actually copes with it is great, and the way her relationship with the Obin evolves over the course of the book is quite fascinating and feels very natural. The thing I liked best about this book was that I finally feel like I understand the Consu. They're this super-powerful race of aliens - far and away more powerful than anyone else in the galaxy - but their motivations are completely incomprehensible and sometimes felt like a deus ex machina. Towards the end of the book Zoe actually has a conversation with one of them, and as a result of what is said the rationale behind the Consu's actions became clear (if incomprehensible - but they are an alien race and I think that incomprehensible is ok) and all of a sudden their actions over the course of the three books suddenly felt consistent.
This was a great book, and one that is going to stay on a shelf somewhere for Elli to discover - sort of the same way I discovered my grandfather's stash of Heinlein novels.