The Mote In God's Eye

Image 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, not until a light sail probe enters a human system carrying a dead alien. The probe is traced to the Mote, an isolated star in a thick dust cloud, and an expedition is dispatched.

In the Mote the humans find an ancient civilization - at least one million years old - that has always been bottled up in their cloistered solar system for lack of a star drive. The Moties are welcoming and kind, yet rather evasive about certain aspects of their society. It seems the Moties have a dark problem, one they've been unable to solve in over a million years.

Related: The Gripping Hand (Larry Niven & Jerry Pournelle, 1993), Outies (J.R. Pournelle?, 2010)

Wikipedia, Worlds Without End, Richard Harter,

By the book from Adlibris