I had to pop over to my bookshelf to check out some of my old books. Here's what I recommend. You can wiki and look up the books on Amazon for the plot synopsis.
1. Metro 2033 (Post-apocalyptic Scifi)
This book is amazing, I bought it after playing the game and it by far trumps it's digital brother. It's actually a Russian novel, but you can find English publications on Amazon and Ebay. It's well worth the time to read it.
2. The Hitch Hikers Guide to the Galaxy. (Comedy, Scifi)
The One, The Only, the timeless work by Douglas Adams. This book is wayyyyy too funny and creative to be passed up by anyone. The movie was great, but the developers trimmed so much out of it.
3. The Starcraft Archive: An Anthology (Scifi, Space-Western-ish-)
Starcraft: Liberty's Crusade
Starcraft: Shadow of the Xel'naga
Starcraft: Speed of Darkness
These are great reads if you're a Starcraft fan as well. These go over the plot in the game (first SC) and it's a more detailed look into the key characters of the Universe. The books start in the first moments of the Sons of Korhal and how the Terran Confederacy was overthrown (This takes place when the Protoss are unknown and the Zerg aren't anything more than a "rumor").
Anyway, those are a few I recommend, they're all great reads. I can recommend more if you'd like.