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.

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!

Not sure about the length, but Vernor Vinge's Rainbows End is one of my favorite works of speculative fiction that really aged well so far.

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)

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.

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

"Short enough to finish in a day" seems pretty tough for me, but maybe I read slowly.

Short story books are good for casual reading in short sessions. Robot Dreams by Asimov, or Welcome to the Monkey House by Vonnegut. I used to carry each of those around and read a short story while waiting at a restaurant or at the DMV or whatever.

I really liked Altered Carbon. Approachable sci fi with drugs, violence, sex, politics, and of course high tech ideas like flying cars, AI hotels, digital consciousness.

The Lady Astronaut series by Mary Robinette Kowal. The first book is called The Calculating Stars. Basically, an alternate history where (spoiler for the opening chapter)

spoiler

a meteor wipes out the east coast and kick-starts climate change, causing the Space Age to start 10 years early.

It follows a Jewish computer (a woman who literally runs calculations for NASA, as seen in Hidden Figures) who wants to become an astronaut, and her struggles with the racism and misogyny of the 1950s.

Best? Hard to say. But favorite?

Galactic Pot-Healer by Philip K. Dick. It's quite short, like many of his books, and you could absolutely knock it out in a day.

I read a lot of thriller/mystery books. Almost anything by Frieda McFadden is a great read and are quick weekend reads.

The Handmaid's Tale

We

Nightfall

The Terminal Experiment

Raymond Chandler's novels, esp The Lady in the Lake

The Pirx the Pilot stories, 8 in 2 volumes

Dashiell Hammett's The Maltese Falcon and The Glass Key

2nd the Hitchhiker's Guide and they're easy to rejoin

A A Fair's novels are short and have odd western us lore in them, one has a great way to bet in Vegas, others name spots in Mexico, they were Gardner's fun books that he liked to write more than other series.

I was entranced by the Aubrey/Maturin series by Patrick O'Brien. Sailing ships, adventure, and a little romance.

Lots of great recommendations here. I'd also add Curse of Chalion by Lois McMaster Bujold. Her Penric novels are quite fun, too.

The Fifth Season may take a bit longer than a day, but it’s worth it.