Questions for a Soldier was initially a chapbook (that is, a short story published in a minimally bound book format) that was released by a small press to further the story of Old Man's War. I'm not sure exactly when it was released, but its chronology in the universe of the series is between books one and two. I didn't know about it until I finished book three, and I'm glad that I read this book after finishing that one, since an event that happens in this story is relevant to what happens in book three. I won't spoil it for you if you want to check it out for yourself, but if you get it in your mind to read it, look for it in an e-book format instead. If you buy it that way, you're out a buck; if you try to get it in the chapbook format, you'll be out one hundred bucks.
The story isn't really much at all. It's written as a transcript of an interview with John Perry, who's taking questions from an audience that wants to know more about him and the CDF. Much of the story references events that take place in Old Man's War, and foreshadow what's going to happen in The Last Colony, so if you've read those books, then a lot of what's here is just restating what you already know. It's not essential by any means, but you still get a bit more about the events that happen to John Perry, and more insight into the CDF. It's a curiosity at best, and I would only suggest it to completionists like me.