This is such an impossible question! I tend to read trilogies and series set in contiguous universes so picking an individual book is a challenge. I am a huge fan of The Wheel of Time series by Robert Jordan but would be completely unable to pick a single one of the 13 book series as a favorite. I love Douglas Adams and the Hitchhikers Guide to the Galaxy but again it is a series that would be difficult to pick a favorite of the series. Then there is Terry Pratchett and his entire Discworld universe of satirical novels, all of which interlink and are hilarious.

If you didn't guess yet I read fantasy/sci fi novels mostly when it comes to pleasure reading. I also do a good bit of non fiction reading for research and self education....

Can I pick... maybe 3?

The Salmon of Doubt by Douglas Adams
I love this book because it is incomplete. Douglas Adams died with it in its very early stages. Following his death his publisher- with help from family, friends, and anyone else that had pieces of his work compiled them, picked out their favorites, and turned them into a compendium of sorts. The book contains short essays, musings, and speeches he wrote as well as the incomplete copy for his unfinished novel by the same name- The Salmon of Doubt. It is a really great book that gives a lot of insight into his creative process and shows off some of his lesser known work and skills.

Thief of Time by Terry Pratchett
This is a satirical tale set in Discworld. It plays with the idea of Time as something humanity manipulates simply by being. It introduces us to the "Auditors" who verify and catalogue everything in the universe- and hate all sentient life for its incessant ability to change its surroundings and constant activity. It makes the auditing more difficult of course. The book follows several characters. One who has been tricked by the auditors into making a perfect clock that measures the tick of the universe and several others who understand that building such a device will in effect freeze time in that moment indefinitely and wish to prevent that from happening. Along the way we get to meet various deities and sages. All of whom offer up various absurd truths and wisdoms. All in all it is one of the best examples of satire I've read with a lot of subtle connections to other Discworld novels that make it that much funnier. Make sure you read the occasional footnotes.

The Resilient Farm and Homestead by Ben Falk
This one is basically a textbook. It is so dense with permaculture knowledge and skills that it will probably take my whole life to properly absorb the information it is offering. It has been instrumental in some of the things we are attempting to do on our property and guided some of the big decisions we've made about what to prioritize in how we manage our land to increase biodiversity, resilience, and continually improve it so that we can harvest useable food and other products from it while increasing its productivity and health over time!