The Hitchhiker's Guide to the Galaxy — Douglas Adams
The answer to life, the universe, and everything is 42. This is among the most important facts in this book and also the least surprising thing in it. Adams wrote five books in the Hitchhiker's trilogy and managed to make the entire concept of existence into a sustained absurdist joke that remains, 45 years later, the best argument for reading science fiction that has ever been made. The Guide itself — a fictional encyclopedia that the book quotes extensively — is funnier than most actual books.
A Short History of Nearly Everything — Bill Bryson
Technically nonfiction. Practically one of the funniest books about science ever written. Bryson explains physics, chemistry, biology, geology, and the history of scientific discovery in language so clear and warm that you will feel bad about every science class you ever found boring. Contains more unexpected details about things you did not know you cared about than anything else on this list. Includes the information that the unit of measurement for nuclear particles was nearly named the 'outhouse.'
Good Omens — Terry Pratchett and Neil Gaiman
An angel and a demon who have lived on Earth long enough to like it team up to prevent the apocalypse because they don't want to go back to their respective home offices. Pratchett and Gaiman wrote this together in what appears to have been a competition to see who could be more casually brilliant per sentence. The footnotes are as good as the text. The characters are more sympathetic than most real people. The book ends correctly.
Catch-22 — Joseph Heller
The defining document of bureaucratic absurdism. Yossarian is a World War II bombardier who wants to be declared insane to avoid flying more missions. The catch: wanting to avoid a dangerous situation is proof of sanity, which means he must fly. Every attempt to escape the war reveals a new layer of the same recursive trap. Written in 1961 about the 1940s, it remains an accurate description of how large organizations function.
My Year of Rest and Relaxation — Ottessa Moshfegh
A wealthy, beautiful young woman in New York City decides to spend a year asleep, medicated into unconsciousness, with minimal interaction with the world. The book is simultaneously very funny and very disturbing, and the two qualities are inseparable. The narrator is not a reliable or sympathetic person. The book is riveting anyway. Moshfegh's deadpan is one of the best things currently happening in American fiction.
Surely You're Joking, Mr. Feynman! — Richard Feynman
Nobel Prize-winning physicist Richard Feynman's memoir consists primarily of anecdotes about him teaching himself to pick locks, joining a bongo band, working on the Manhattan Project while also cracking open classified safes for fun, and generally being the most interesting person in any room he entered. The physics is present but secondary. The stories are primary. Feynman had a gift for explaining complex ideas simply that extended to his own life.
Confederacy of Dunces — John Kennedy Toole
Ignatius J. Reilly is one of the greatest and most insufferable protagonists in American fiction — an enormous, medievally philosophical man living in New Orleans with his mother, convinced of his own genius and the world's comprehensive wrongness. The book was published posthumously after Toole's mother spent years submitting it. It won the Pulitzer Prize. Ignatius would find this appropriately confusing.