For sure, nobody needs to worry about reading any of the books other than the original. The story begins, happens, and ends, all in one not-very-long novel.
After that novel is a whole series of novels that follow Ender into distant futures and places, as he literally runs from his past to keep it from defining him. The first one, Speaker for the Dead, is very recommendable for follow-up reading. The others I would only recommend if you've already decided that you want to read more of the series.
However, there is also a different set of sequels, and the first of them is the first book I would recommend to anyone who wanted to read more: Ender's Shadow. (Like Ender's Game, it won both a Hugo award and a Nebula award.) It takes place during the same time as Ender's Game, but from the perspective of one of the secondary characters. This second series of sequels generally takes place on Earth in the time shortly after Ender's Game, and follows Ender's siblings and the other Battle School characters. Like the first series of sequels, I won't go out of my way to recommend these to people who haven't already decided they're interested in reading them.
AS I write this comment, I learn that Orson Scott Card has been writing prequels: a trilogy in 2012-2014 (Earth Unaware, Earth Afire, and Earth Awakens) describing the First Formic War (which took place ~50 years before, and is heavily referenced in, Ender's Game), and a not-yet-complete trilogy since 2016 (The Swarm, The Hive, and the not-yet-published The Queens), which appears to be about the actions that were taken after the First Formic War to produce Ender. The reviews on Goodreads are promising; I might read some of these.
While I’m sure it sums up well as a novella, as pre-teen, the “big reveal” probably hit with more force by being in novel format. It reads like a typical “coming of age” adventure, and then…suddenly isn’t. Although I haven’t read it since then, I think there was a fair amount of character development, which also increases the impact of the reveal since you’ve been given time to care about the characters and relate to them. Also, as a kid who loved strategy games, the training program was just really fun to read.
While I don’t usually care about spoilers, even knowing that there is a big reveal feels a bit like a spoiler.
After that novel is a whole series of novels that follow Ender into distant futures and places, as he literally runs from his past to keep it from defining him. The first one, Speaker for the Dead, is very recommendable for follow-up reading. The others I would only recommend if you've already decided that you want to read more of the series.
However, there is also a different set of sequels, and the first of them is the first book I would recommend to anyone who wanted to read more: Ender's Shadow. (Like Ender's Game, it won both a Hugo award and a Nebula award.) It takes place during the same time as Ender's Game, but from the perspective of one of the secondary characters. This second series of sequels generally takes place on Earth in the time shortly after Ender's Game, and follows Ender's siblings and the other Battle School characters. Like the first series of sequels, I won't go out of my way to recommend these to people who haven't already decided they're interested in reading them.
AS I write this comment, I learn that Orson Scott Card has been writing prequels: a trilogy in 2012-2014 (Earth Unaware, Earth Afire, and Earth Awakens) describing the First Formic War (which took place ~50 years before, and is heavily referenced in, Ender's Game), and a not-yet-complete trilogy since 2016 (The Swarm, The Hive, and the not-yet-published The Queens), which appears to be about the actions that were taken after the First Formic War to produce Ender. The reviews on Goodreads are promising; I might read some of these.