I wonder if there is a classification of English literature that can tell whether a book is easy to read/understand (good for beginners) or difficult to read/understand (experts)?
I know there is literature for school (specifically designed for learning purposes), but this is rather academic. I'm looking for something general, a database maybe?
I would go through lists of award winners in young adult fiction (also called "YA fiction")[1][2]. Like a Pixar movie, a lot of good YA fiction is layered: entirely appropriate and engaging for children, but with themes, jokes, and content that adults will pick up on that makes it interesting to read at any age. The language is almost always a bit simpler and more straightforward so it would be suitable for young adults . . . or English language learners.
Just to get you started, a few personal YA favorites I've read recently are The Graveyard Book by Neil Gaiman (Newbery winner; also won the Hugo which along with the Nebula is one of the two big awards for best Science Fiction, YA or not) and The Absolutely True Diary of a Part-Time Indian by Sherman Alexie (National Book Award for Young People's Literature and a bunch of other awards). The latter has a strong theme of challenges facing Native Americans today, so you might learn a bit of US culture as well as language.
Also, any book you've already read in your native language is a good book to try in a foreign language, since you already know the plot.
One other author that comes to mind is Ernest Hemingway, who is famous for using very short sentences and straightforward writing.
[1] https://en.wikipedia.org/wiki/Young-adult_fiction#Awards
[2] The Newbery Award, while technically for "children's" books, runs over into YA fiction. It's also older, more well-known, and prestigious than any of the YA awards, since YA wasn't really considered a genre until more recently.
