Fanfiction is my go-to when I'm sick, or depressed, or really tired. Nothing beats its combo of easy-to-read and wildly absorbing.

Otherwise, I'm partial to cozy fantasy, like The Goblin Emperor by Katherine Addison.

Also the Murderbot books by Martha Wells never fail me.

Also... older books, with their wordiness and long sentences and lack of fear of semicolons, can be great for this. Virginia Woolf's books for example are so dense, and the atmosphere that creates is sublime, but the way she writes somehow makes her wordy prose also really easy to read. The sentences just kind of tumble you along. I love it.

Caveat being that older books, including Woolf's, contain shit like casual racism and sexism and etc, and sometimes I have more capacity to overlook that than other times. Which is one reason I love when modern authors write in a more old-fashioned style, like in Jonathan Strange & Mr Norrel by Susanna Clarke - it lets me have my cake and eat it too.

Poetry is also great a rainy day, including old poetry. It's underrated these days. I think it's partly from the pervasive modern idea that poetry is automatically "cringey", and partly from the elitism and other -isms among the Poetry Establishment^tm, and partly from English teachers taking the fun out of it. But you can rediscover poetry, just like you can rediscover a love of novels after highschool steals it from you for a while.