The DU Lounge

I recommend Issac Asimov's Foundation trilogy. In Foundation, young Hari Sheldon developed a new branch of mathematics called Psychohistory which predicts the future based of large populations. He convinces the Galactic Emperor the empire will fall within 50 years followed by 10,000 of a dark age. He convinces Emperor to establish a Foundation on a planet on the outer edge of the galaxy they can save all the scientific knowledge of empire and reduce the dark age to 1,000 years. During this 1,000 year period, the Foundation will face many challenges.

The original series consisting of three books but Asimov later added four more books and after his death his estate authorized prominent scifi writers to continue the series. The first three novel are all you need to read but if you really get into the series then you'll enjoy the rest of the series
Foundation
Foundation and Empire
Second Foundation

Another series I like are the Ben Bova's Mars novels which deals with the exploration and colonization of Mars in the late 21st century. There are no aliens, flying saucers, blasters. etc. like some of the pulp scifi novels. The novels are more about the characters of the people exploring Mars and their interaction with each other.
Mars
Return to Mars
Mars Life

If you want a really old scifi books, try When Worlds Collide and After Worlds Collide. It's a story set in the late 1930s about a rogue planet with a smaller planet with an Earth-like atmosphere, headed towards a collision with Earth. The larger planet will destroy the Earth and continue on its journey while the smaller planet will remain in the solar system. A small group of scientists and industrialists join together to build a rocket that would take a small number of humans to the small planet to live and continue human existence. After Worlds Collide tells their story of survival on the new planet.

All three series may be available in your local library.