The book is about Raul Endymion--a man in his late 20s who is chosen by the poet Martin Silenus to find and rescue Brawne Lamia's daughter Aenea from the Pax government. The book has a lot of religious politics in it which I found really interesting. However, it was a little bit slower read through a lot of the book because of it. The story is a continuation of the Hyperion Cantos. I was a little surprised that there was another book after The Fall of Hyperion because all my questions were answered at the end of it. Dan Simmons weaves another great story out of all the elements from the first two books. The Technocore, cruciforms, and farcasters all play an important role in this book. I like how masterful Dan Simmons is at telling the story. There were times that I got so lost in the scenes that I could actually feel when he was describing how cold it was or how the gravity affected their bodies.
The android from the first book, A. Bettik, was also a prominent character in this third book. I really enjoyed reading about him even though he was such a minor character in the first book. He was no more than a passing mention really, but he is such a likeable character and has his own unique story that I'm glad Simmons made him a major part of this story.
Like Hyperion, Endymion ends in the middle. In other words, there is no resolution so I feel like I need to read the last book of the series pretty soon. So be warned, if you pick up the series, it's probably best to pick up all four books at the same time, or pick up the first two books at least (the last two happen ~270 years later so it's not necessary to read them all at once). This series is definitely on my top 10 books of all time so far. I have really enjoyed the series and think the quality of writing is superb.