"Foundation's Edge" by Isaac Asimov

This is the fourth book in the Foundation series by Isaac Asimov, and the first after the original trilogy, published several decades later. The Foundation is in its 500th year of the Foundation Era, the Seldon plan is going great, the second foundation is destroyed, and a new empire is to rise in 500 years from the Terminus core. But is it all so great? Trevize, a councilor on Terminus thinks that the second foundation still exists and guides the Seldon plan, and exactly because the plan has been so successful, he shows it as a proof that the second foundation is influencing it. The mayor of Terminus exiles him to act as a bait for the second foundation, along with a historian searching for the mystical Earth, Janov Pelorat. At the same time on Trantor, the traditional seat of the very alive Second Foundation, Gendibal, the youngest but the most ambitious Speaker at the table shocks the council by saying that there is another force that makes sure that the Seldon plan goes as planned and maybe wants it for some own purpose. He shows a changed mind of a Hammish woman, Novi, which was beyond the skill of anyone in the Second Foundation.

Gendibal follows Trevize and Pelorat in their search for Earth, which they think is the planet Gaia, in the Seyshell sector. It turns out Seyshell sector was settled directly from Earth, by humans who hated robots, which were used in all the earlier extraterrestrial settlements. They formed a planet Gaia, by working on their telepathic abilities and becoming one conscious being, together with all the humans on the planet, all the animals, life forms and inanimate objects like the planet itself. Here Travize has to decide a stalemate among the first foundation, second foundation and Gaia, on which will depend who will rule the universe. Travize decides on Gaia, mostly because that was the only reversible choices, the other two including a destruction of Gaia. Towards the end Travize discovers that some of the humans on Gaia may actually be robots from the pre-imperial past of humanity, who, now thousands of years old, are still trying to guide humanity as helpers and teachers.