"Existence", by David Brin
Science Fiction
http://www.amazon.com/Existence-David-Brin/dp/0765303612
Set in the year 2030, the book poses a deceptively simple setup. An artifact not of this Earth is found, a sort of message in a bottle. The message it contains is simple; "Join us." We humans react to the discovery in, well, a very human manner, with fear, hope, greed, vanity, and more than a little panic.
The story itself is huge, sweeping across multiple generations, with dozens of characters, covering ideas related to death, birth, and everything in between. It's a book full of Big Ideas and astounding plot twists. Because of all this, it can also be a difficult read, so be forewarned. Characters come and go as their lives become more or less relevant to the story. Several of Mr. Brin's favorite ideas and themes pop up within the story, so if you've read his nonfiction articles or blog, there are a few chapters that will sound familiar. If not, well, there's an incentive to visit
http://davidbrin.blogspot.com/.
Note: The story lands in a niche called Mundane Science Fiction, a flavor of SF story that has to play by the currently understood rules of the laws of physics. So, no vast interstellar empires, faster than light spaceships, or similar stuff. Still, the book packs in some things that you wouldn't expect under these ground rules...