The The Gentle Giants of Ganymede ended with the lost Ganymean expedition going back into space in an attempt to find the new civilization which they believe, from their joint work with Terran scientists, the last of their kind left the Solar System to found at another star system. Everybody believed it a pretty forlorn hope, but nobody said so.
However, not only is the "Giants' Star" found to exist, but certain of its inhabitants turn out to have been involved in much of the development of Earth's history, particularly its more sordid and violent episodes. And for a change, homo sapiens is revealed as being not so bad after all, as we've often been led to think.
Much of the book involves interaction between us and them carried out via an alien Virtual Reality technology that couples straight into the neural centers and operates across a network of interstellar dimensions--not bad, I thought, having been written in 1979-80, before PCs were heard of.
As far as I was concerned, this was the end of the "Giants" series. Others thought differently, however, and later, under pressure of requests to take the story further, I wrote Entoverse and Mission to Minerva.
See Dr. Attila Torkos' Giants Chronology for a timeline, recently updated to include all five novels. Click here to view the HTML version on-line, or right-click here to download an Adobe Acrobat PDF file, nicely formatted for printing.