Orson Scott Card's

Ender's Game

Speaker For the Dead

Xenocide

Original trilogy. There is a critical lesson(s) in each book, imo.

Ender's Game is primarily about leadership

Speaker For the Dead is primarily about cultures and cultural differences and how they affect relationships

Xenocide is primarily about ethics. It's built around a set of "wicked problems".

All three should be military and academic required reading.