Foundation's Edge by Isaac Asimov (1982)

Imagine being in a position where you are begged by fans and forced by your publisher to write another book. This book is the sequel to a series that was voted as the Best All-Time Series (a one time award that was never repeated). And the book that you write becomes a bestseller for the first time in your career and wins awards from the industry. This is what happened to Isaac Asimov and the book - Foundation's Edge. The book was published in 1982 (and since the Second Foundation was published in 1953, this one was a long time in coming). It was much thicker than the first 3 books of the Foundation series and was also a bestseller on the New York times best-seller list (and given that Asimov was a prolific writer, it took a long time for him to reach the best-seller list).



Foundation's Edge takes the timeline of 500 years after the birth of the Foundation, half-way into the 1000 year period predicted by Hari Seldon for the formation of the next Galactic Empire. The Mule has long been gone, and the distortion introduced in the Seldon Plan by the emergence of the Mule has gone away (due to the careful attention of the Second Foundation). So too has the thought of the Second Foundation, ever since the First Foundation believed it destroyed the agents of the Second Foundation on Terminus.
However, not everyone believes this. The all powerful Mayor of Terminus (and consequently the ruler of the Foundation), Harla Branno, is not sure whether the Second Foundation is truly gone. Similarly, there is a young and impetuous Councilman, Golan Trevize also believes that the Second Foundation exists and is willing to ask this in open Council even though the Mayor is not willing. He has already confided his feelings to another young Councilman Munn Li Compor, who has however disclosed all this to the Mayor. The Mayor is all powerful right now, since there was a opening of the vault and an appearance by Hari Seldon that strengthened her position immensely; she is able to arrest Trevize and send him to exile. She wants him to search for the Second Foundation, while apparently searching for the obscure hidden planet Earth with a older researcher Janov Pelorat. He is given the most advanced ship that the Foundation has, a gravity ship that takes its energy from the gravity forces in space.
Simultaneously, there are more political battles happening in Trantor, the home of the Second Foundation, where 2 upstarts are competing to be the next one to take over as First Speaker, with Stor Gendibal trying to warn about the belief that there is some power stronger than the Second Foundation. In this he is opposed by an opponent Delora Delarmi, who wants to diminish him since that would leave the path clear for her to be the next First Speaker.
In the midst of all this, the entire sequence is actually being guided by the planet of Gaia (Wikipedia), an extraordinary planet where everything is conscious, and part of one. Everything on the planet has one thought, one consciousness, and together the planet wields immense power, even stronger than the Second Foundation. The Gaia concept is also non-threatening in the sense that there is no ambition, but there is a touch of vanity since Gaia believes that the entire Galaxy should be like Gaia, with all elements of the Galaxy being part of one. The reason why Gaia is manipulating everyone is that it wants to bring the situation to a head, and make Trevize decide the future of the Galaxy (he apparently has a very strong intuition, and Gaia wants to him decide whether the future of the Galaxy should be based on the physical power of the First Foundation, or the mental control of the Second Foundation, or the wholeness of Gaia). Whatever decision he makes will be implemented by Gaia.