What are some good books about space exploration/colonization/first contact?

I have always had an interest in astronomy and space exploration, and recently I saw the documentary "Alien Planet" which was based off of "Expedition: Being an Account in Words and Artwork of the 2358 A.D. Voyage to Darwin IV" by Wayne Barlowe, and I like the idea of the first encounter with an alien lifeform.

I also enjoy the Halo series (hence the username.) So basically, what I'm looking for are some books that would be a fictional (obviously, as we haven't made contact with aliens yet) representation of how humanity might come across an alien civilization or something along those lines.