Babel-17 by Samuel R. Delany
In the future, humans have colonized many star systems. Currently, the Alliance is engaged in a war with the Invaders, who, despite the name, are also human. The Alliance has intercepted many dispatches in a code they can’t break. They’ve labeled it Babel-17. Desperate, they turn to the inter-galactically renowned poet Rydra Wong to help them decipher it.