Dune by Frank Herbert. If you've never heard of it, you're in for a real treat. If you like Ender's Game, then I think you'll appreciate the Dune series. Don't ask me why, because I can't pinpoint a reason. I know that's not much help, but . . . give it a try. I don't think you'll be disappointed.
As far as your obvious enthusiasm for Ender's Game . . . I can say I share it. I haven't read the side stories, but the main series had me hooked. I wish they didn't have to end. I always enjoyed books about humans encountering alien cultures, not just encountering aliens, but actually having to deal with the differences and the consequences of misunderstanding those differences. If you like that aspect of Ender's Game, try The Sparrow, by Mary Doria Russel. There is a sequel, Children of God. (see the thread under authors, I think)