Forever War is a fantastic take on future interstellar war.
Forever Free starts out really slow and at times it almost reads like fan service. There are good parts and also parts that are a bit too close to The Hitchhiker's Guide to the Galaxy for my taste.
Forever Peace is also a great novel inspired by modern wars and the human condition.
Now I'm reading Masters of Doom by David Kushner. The first chapter is excellent and it is probably worth reading even if you aren't interested in video game history.