Cradle! Or better, the cradle series. It's a sort of adventure story in a fantasy world.

Pretty much anything in the "Known Space" series by Larry Niven (et al - there are works by some other authors in that space).

Rn I'm currently rereading The Inheritance Cycle, it's fantasy, but it goes very in depth, there are your different races, elves, "orcs", dwarves, you got dragons, there are different languages that the author made, its very good. Of course I might be biased since I'm rereading it rn lmao

Edit: I did not read the bit about reading it in a day. I guess you could if you read fast

If you're into short stories the Illustrated Man by Ray Bradbury is a good one.

And while I didn't read much Issac Asimov myself my wife, who loves reading but dislikes sci-fi, read one of his books (Foundation) in a day and said he's an excellent writer.

I reccomend hopping on !chat@literature.cafe

Any early Alistair MacLean...

Guns of Navaronne

Where Eagles Dare

When Eight Bells Toll

Night Without End

Puppet on a String

Louis Lamour's westerns are complete popcorn and fun to read

C. S. Forester's Horatio Hornblower books

The Locked Tomb series is refreshing. It’s weird, it’s fun, it’s dark, and it’s trash, but it’s trash that the author is having fun with.

Discworld is also just amazing

Two for you:

《The Wild Girls》 - Ursula K Le Guin

《Piranesi》 - Susanna Clarke

And if you read fast I reckon you could do China Miévilles 《The City and the City》 or Tade Thompson's 《Rosewater》 in a day.

Edit bonus: anything by Douglas Adams.

I would recommend checking out audio books as a medium for reading. It allows you to increase the speed to whatever works for you, so 2x for me, and listen to a lot more in a day. It also frees you to listen at any times you have nothing cognitive happening, so dishes, washing, cleaning, etc.

As for single day books, the first book of the Bobiverse series by Dennis E Taylor. I loved the whole series including the recently released 5th book and the first is only 9.5 hours at normal speed, so about 4.75 at double speed.

Also All Systems Red is the first book in the Murderbot series by Martha Wells. The perspective of a SecUnit, a type of sentient cyborg, which has hacked its own programming and removed its limiters so it can act freely. This means no guard rails, no rules, no limits, which results in lots of TV shows being watched and avoiding humans. It is snarky, fun, and interesting. It comes in at 3.5 hours normal time, so 1.75 at double speed.

Recommend high quality short stories. Edgar Allen Poe has a collection that is some of the most thrilling, mysterious and fun, imaginative, adventurous, grotesque and other depending on the story. https://www.amazon.com/Edgar-Allan-Poe-Complete-Collection/dp/1453643141

Robert Louis Stevenson was also a fantastic writer of short stories.https://www.amazon.com/Complete-Short-Stories-Robert-Stevenson/dp/030680882X

I like short stories sometimes as I can't commit to a larger read.

Others may have mentioned it (happy to see Terry Pratchett getting a lot of love), but would definitely recommend anything by Vonnegut! Love his writing style and his approaches to humor and world building. Slaughterhouse Five is a great one, as is Sirens of Titan.

Also, not certain how well they hold up, but I really enjoyed the Redwall series back in the day! I was much younger at the time, though.

"The Theory and Practice of Oligarchical Collectivism" is a hell of read, as well as "The Navidson Record".

But "The Necronomicon" is my favorite fictional book, I think.

I second someone else suggestion: the murderbot diaries. It's great.
Most of the books people here are recommending are fairly lengthy, but you can get through the first murderbot book in a dedicated evening.

Completely ignoring your "short enough to finish in a day" instruction, try out Worm

I'm waist deep in The Dresden Files right now (just started Turn Coat, book 11 of like 20 and counting) and it very quickly became one of my favorite series I've ever read. Jim Butcher has woven a web of a story where every little detail is a foreshadow that often won't pay off until two books later, it's incredible.

Prior to this I read The Expanse and that one also comes highly recommended. It's one of the most believable space operas I've ever read. I also hear the TV show is good, no idea, never watched it.

I have just the book for you!... Ah, finish in a day, nevermind.

When I was young, I read Diane Duane's The Young Wizards series, and I remember I loved it. Also Artemis Fowl, Sherlock Holmes, and The Inheritance series (C. Paolini). As an adult, I've read the LotR series which I highly recommend. Also, The Expanse series, 1984, Chronicles of Narnia.

Short enough to finish in a day...hmm that's tough. Maybe Screwtape Letters by C S Lewis? The Martian. Lots of short stories out there by Isaac Asimov!

"Best" often is a literary work that can be slow to read and/or very long. You want stuff that is short and quick, which is fine, I read a lot of fanfiction for that purpose. But I'm going to recommend Pohl and Kornbluth's "The Space Merchants" and their other short novels from that era (1950s). Their cynicism is absolutely prescient. The Space Merchants is about a world run by advertising agencies. A quick read while hard hitting.

A couple of my favorite books are probably longer than a day’s read:

• Sirens of Titan - Kurt Vonnegut (319p)
• The Watermelon King - Daniel Wallace (240p)

Robert Silverberg's "The man in the maze" is a cool science-fiction book based on the Greek play Philoctetes. Iirc it's a very short story (maybe about one or two hundred pages), I don't remember the exact length but I recall reading it in one sitting. It is a very character-driver story where the "maze" itself is an allegory about mankind, isolation and disability, but it is very much enjoyable as a casual read as well.

The protagonist ("man in the maze") is an astronaut who has been somehow cursed to always radiate its emotions in such a way that others, even his family, find repulsive, so he self-exiles to a remote and long-dead planet to live the rest of his life in isolation. But when an alien species makes hostile contact with humans, he is needed again, as his "curse" is the only way to properly communicate with them and maybe convince them that humans are sentient beings and thus their equals.

The Broken Earth series, Enders game series (the first 5 books about Ender), American Gods, An Absolutely Remarkable Thing and the follow up A Beautifully Foolish Endeavor, The Kingkiller Chronicle (we've been waiting 10+ yrs for the final book 3, some folks are pretty irked atp, but it will be ok). If you want YA beach reading, anything by Seanan McGuire / Mira Grant for easy fun books about fairies, cryptids, and zombies.

My favorite easy fiction that helps me unwind is Agatha Christie mysteries. There is a reason she is the greatest mystery ŵritwr of all time. She sets up compelling situations and makes her way to a damn satisfying conclusion by the end.

A few of her shorter but still excellent stories: The Secret Adversary N or M The Unexpected Guest 3 Blind Mice Halloween Party Murder of Roger Akcroyd

Also if you like Mysteries I have to plug my all time favorite: 7 1/2 Deaths of Evelyn Hardcastle

It is a great mystery in which the protagonist wakes up with no memories and has 8 chances to solve a murder.