WARNING: If you want to read the first book of this series (Cretaceous Sea) without a spoiler, skip this review.
After its light treatment in his first novel, Cretaceous Sea, Will Hubbell has followed with a sequel that really digs into the philosophy and paradox of time travel. Sea of Time looks at the present-day and future of an age engendered by the incidents detailed in the first novel.
Alteration of the present by the past is an important concept in the story. At the end of the first story, Constance "Con" Greighton and Rick Clements are rescued by people from the future, and taken to their own past, to gold-rush California, where Rick and Con are married. (Con thus not only founds the family fortune in a California gold mine, but also becomes her own great-great-grandmother.)
What should have been a "happily ever after" ending to the first novel instead becomes a launching point for multi-dimensional murder. The evolved homo perfectus society that created the time machine is worried about awareness of the possibility of time travel by unevolved people in their past. Con is approached by one of them who warns her.
"They may alter your reality instead... If they were to change your past," replied Sam, "from that point onward, the resulting reality would be the only one you knew. They might erase all who stand in their way, all who are precious to you."
Minutes later, Con learns that Rick has been shot to death. In the ensuing weeks, her only son also dies of cold and starvation. Having lost all she holds dear, she agrees to come to the future with the "Kynden" Sam, to work against those who supposedly arranged her husband's murder, in order to "undo" his and her son's death.
In the future, however, Con finds things not quite as Sam presented them to her. There are three groups of people in this "perfected" future world: homo perfectus or "fecs", home sapiens or "sapes", and the Kynden, who sit between the two, and want to eliminate both species from 27th-century Earth. Sapes occupy something of a plantation slavery role in this society, due to a virus-imposed addiction to kana, a drug that is only available from the fecs.