A few people have already recommended The Expanse by James S.A. Corey. Probably the best book series I've ever read. Any Heinlein, but especially Starship Troopers and The Moon is a Harsh Mistress. Frontlines series by Marko Kloos is great mil SF. For a little bit more wild fun, the Undying Mercenaries series by B.V. Larson is a treat. Also, Mike Kupari's Sins of Her Father series is excellent fun. Pretty sure Mike actually plays Infinity. For less direct SF, Mike Kupari and Larry Correia collaborated on the Dead Six series, which is also quite fun. Larry does play Infinity and posts on WGC frequently. All of Larry's other works are amazing as well.
The David Brin Uplift war series is good - especially if you play tohaa (the "uplifting" of lower species to sentience is remarkably similar to the tohaa "exhaltation"). The deeper you go into the whole SF genre (well, at least 30-40 years), the more you will see where certain concepts in the game come from.
Oh, David Brin! I enjoyed his Kiln People. The "Dittos" in that book heavily influenced how I originally envisioned LHosts when I first heard of the concept.
Revelation Space series by Alastair Reynolds. Listened to them as audiobooks, suspensful stuff. Also everything by Vernor Vinge.
For something completely different, check out Eric Flint's 1632, Ring of Fire. The first book can be downloaded, free -- legally, from Baen's webpage.
