Sundiver is a novel written by David Brin in 1980. It is a self contained story that happens several decades before the other books in the series.
The events that made Jacob Demwa a hero also left him broken inside. His attempt to live a peaceful life training dolphins and healing is interrupted when mysteries threaten the political future of humanity. Soon Jacob Demwa is called back into his life as a high-stakes detective by powerful aliens, even those he called friend.