The aliens are really alien - not just green or pointy eared, and Niven and Pournelle create a believeable society - both the alien society and the future Empire of Man.
In the year 3016, the Second Empire of Man spans hundreds of star systems, thanks to the faster-than-light Alderson Drive. No other intelligent beings have ever been encountered, until one day an alien craft enters space near a human occupied system. All aliens aboard are unfortunately dead. The probe is traced to the Mote, an isolated star next to a nebula, and an expedition sets out.
In the Mote the humans find an ancient technologically advanced civilization (the Moties) that has always been bottled up in a single solar system. It's not that they can't make a star drive but they are in an unfortunate location in space that makes a star drive impossible - they need to know one more secret about their solar system, which they can't know as it has to be seen from outside..... The Moties are welcoming and kind, readily helping the humans - who are holding the secret of the star drive - yet it rapidly becomes clear that there is something the Moties are hiding from the humans.
The aliens and their civilisation are the stars of this book and as Niven and Pournelle are great science fiction authors, all the details are right. The writing itself is a bit on the clunky side (like Dick Francis in space) but the novel covers some extra aspects of the aliens and First Contact - biology, sociology, art, economy and religion are all examined in this book.
Go read it - you'll be awful glad you did......