Hitchhiker's Guide to the Galaxy.
It's such a fun book to read, the movies do it no justice.

A Thousand Splendid Suns by Khaled Hosseini - I don’t feel I could give a synopsis that would do it justice, just a must-read!

Unwind. Although it is of a bleak future, it has always drawn my attention to the possible future we could unfortunately have. Thought provoking I guess.

Squire Haggard's Journal by Michael Green. He is living vicariously through me.

The Belgariad. A set of five books about a fantasy world with what I still think is a unique take on magical ability. The story is archetypal, good guys have to go and stop the bad guys, but I still like it, even 10 years after I read it. I think I'll go read it again. There was also a set of books called the Edge Chronicles that was super interesting to read, just to understand how the world works. For a scientific law in the Edgeworld, Cold rock rises, and hot rocks sink, and they use this to fly Sky Ships across the world.

I have lots of favourites but one trilogy that I think don't get the attention it deserves is C.S. Lewis's sci-fi work Cosmic Trilogy consisting of the books Out of the Silent Planet, Perelandra & That Hideous Strength. It's a story with elements of sci-fi and various mythologies (classical and christian) bundled up in some lovely quaint philosophy, plus a while slew of crazy characters and monsters and a plot for world domination.