Older books will have more difficult prose generally speaking. There are plenty of 'modern classics' of the 20th century you could go with.
John Steinbeck - Of Mice and Men, Grapes of Wrath
Ernest Hemmingway - A Farewell to Arms
William Golding - Lord of the Flies
Harper Lee - To Kill a Mockingbird
Joseph Heller - Catch 22

If you enjoyed Orwell you might as well read his other works. I've read 1984, Down and Out in Paris and London, Road to Wigan Pier and a few of his short stories and essays, Shooting an Elephant and A Hanging are interesting short stories that stick out to me.

The books I suggested above will give a good idea of where you'd like to go with your reading.

You could also read a translation of The Odyssey and The Iliad by Homer. There are loads of translations of these to choose from, it might be best to start with a modern prose translation. Homer is widely considered to be the foundation of the 'Western Canon' and generally people considered 'well-read' will have read Homer at some point in their lives.

There is a book called How to Read a Book by Mortimer Adler that teaches you to read 'critically' but he also includes a large reading list at the back of the book that he considers to be great works of literature from the Ancient Greeks up to the 20th century. You could perhaps research some of the works on that list to see if anything takes your interest.