The Mote in God's Eye.

Larry Niven and Jerry Pournelle.

Synopsis:
An alien probe arrives at an Earth colony, fires on the ship sent to investigate it, then tries to dive into the sun. It is recovered, but the crew are dead. Two warships are sent to the system the probe came from to contact the aliens; but there is immediate tension between the military and the scientific community...

Review:
Very enjoyable, especially the gradual revelation of the alien culture, which makes you want to keep on reading to find out what you're going to be told in the next chapter. It's also very believable, in that the military commanders (ie the ones with actual power) are distrusting and see everything as a potential risk but the scientists and sociologists think the aliens are friendly and peaceful.

It's also a very believable depiction of humanity in the future - the level of technology is quite basic in many areas because of several interstellar wars, and the 'Empire of Man' is ruled by the aristocracy. Therefore the political response to the discovery of the aliens is very realistic, which is disconcerting (and a little depressing...) if you're more used to the idealistic 'humanity will become peaceful' approach taken by a lot of sci-fi.

It is quite a huge slab of a book, but successfully keeps the reader's attention throughout.