The original three books of the five book Giants series are a pretty good read. Unfortunately, the time lag between book three and four (ten years), and then the even greater lag between book four and five (fourteen years), make those last two books seem detached from the original trilogy.
I purchased the books as ebooks, and they come as “The Two Moons ( Inherit the Stars and The Gentle Giants of Ganymede), The Two Worlds (Giant’s Star and Entoverse), and then the third/fifth is Mission to Minerva
Confusing when I went to buy. “Get all 3 books in this 5 book series!”
The first two books ( Inherit the Stars and The Gentle Giants of Ganymede) are the most enjoyable, as they are kind of told as scientific mysteries. Inherit the Stars, published in 1977, has this description:
The man on the moon was dead. They called him Charlie. He had big eyes, abundant body hair, and fairly long nostrils. His skeletal body was found clad in a bright red spacesuit, hidden in a rocky grave. They didn’t know who he was, how he got there, or what had killed him. All they knew was that his corpse was fifty thousand years old — and that meant this man had somehow lived long before he ever could have existed.