The movie was sorely disappointing, which was itself disappointing because Ender's Game isn't at all unfilmable (except that the main characters are all children in a really messed-up environment).

It really is similar to Neon Genesis Evangellion in many respects. If NGE had existed first as a novel, it's hard to think that the TV adaptation would be anywhere near as good as what was actually made.

I highly recommend the book, with almost no reservations. It's not very long, and a very stimulating read.

If you enjoy that one, and want to read more, then I'd recommend reading either one or both of the immediate sequels (Ender's Shadow and Speaker for the Dead). Iff you read either of those, and enjoyed it, and want to read more, then (and only then) should you concern yourself with the trails of sequels.