I reviewed quite some time ago when the movie came out. Speaker for the Dead is, in Card's own words, the book he actually meant to write and Ender's Game was meant sort of as a prologue. Card himself is aware of the irony that Ender's Game is by far the most widely read of his works and Speaker for the Dead is left playing second fiddle.
That being said, I think Speaker for the Dead is a great example of how science-fiction as a genre can be serious literature that talks about serious themes which seem to be relegated to''real'' literary fiction more often than not. This isn't to say that Ender's Game doesn't deal with serious topics as well, but I would say that Speaker for the Dead feels much more serious than its predecessor. An analogy I heard once was that Ender's Game is kind of like The Hobbit to the later three books which are more like The Lord of the Rings. They're all within the same universe and share characters, but there's a definite tonal shift between the two. I'm definitely sensing the tonal shift and I feel like Speaker for the Dead, and most likely its sequels Xenocide and Children of the Mind, are going to be more mature than Ender's Game.
Plot-wise the book is set some three thousand years after the events of Ender's Game and the Third Bugger War where humanity destroyed the buggers for good and humanity has scattered among a hundred colony worlds. Ender is remembered as the Xenocide, the worst monster in all of human history, while the most revered is possibly the first Speaker for the Dead who wrote The Hive Queen and the Hegemon which made humanity realize the barbarity of their extermination of the buggers and led them to repent the act. The irony that these were the same person is not lost on Ender Wiggin, who is only in his mid thirties thanks to the time-dilation of interstellar travel, wandering from world to world with his sister Valentine.
Events on the colony world Lusitania have drawn Ender's attention because humanity has made contact with another sentient species, referred to by humans as the piggies. Conscious of their collective sin of exterminating the buggers, humanity has tried to compensate in the other direction by making very limited contact with the piggies and keeping them safe from human contamination. However when one of the xenographers who has been studying the piggies turns up dead, apparently ritually murdered by the piggies, Ender fears humanity may let their more ruthless instincts get the better of them.
Further complicating this is the existence of Jane, a sentient AI created by the ansible network which links the disparate colonies with instantaneous communication. Jane is very aware that humanity has feared the emergence of an AI and has only revealed herself to Ender, the one individual who wouldn't immediately react with fear to her presence. Jane hopes that someday humanity might be ready for her to come out into the open, but she'll need Ender's help to get humanity to that point.
I have to say, despite the disparate plots going on at once, this book is really good. I was engaged throughout the book and although there are plenty of rough corners, I ended up enjoying it. As I said earlier, this is a great example of how science-fiction can talk about serious topics just as well as ''real'' literature. According to Card he just had the idea of someone telling the truth at a person's funeral, his idea for the Speaker for the Dead. He noticed that so often after someone passes on we tend to lie about the person, in the tradition of never speaking ill of the dead. So he thought it would be interesting, if perhaps painful, to tell the truth instead. From this idea has spawned a book that's about so much more than death and secrets and the lies that come with it. There's humanity, what it means to be human, compassion, empathy, and a whole host of other emotions which make this a really interesting book.
As I said, there are some rough edges around the book. One of the things I found really weird about the book was how dated it felt for a distant space future, especially since the book was written in the 1980's. The Brazilian colonists on Lusitania may strongly represent the Brazilians Card encountered during his missionary work there, but I've no way of knowing if they're indicative of Brazilians today or even thirty years ago. There's a major plot point that revolves around the fact one character can't marry another because then he'd have access to all the information she had because husband and wife are considered legally the same. Even for the eighties this feels outdated since it's well past Second Wave Feminism and much of the fights of First Wave Feminism in the early 1900's was to allow women to maintain a distinct legal identity from their husband. It just feels incredibly weird to me that this would be the case in the space future.
Overall I highly recommend this book, and I can say it is popular for a reason. If you've read Ender's Game but for whatever reason never got around to the sequels, I can say at least Speaker for the Dead is well worth the effort. If you haven't read Ender's Game, I would recommend starting there because it's a pretty short read and it will fill you in on a lot of details before you start Speaker for the Dead.