I suggest the Narnia series by CS Lewis. (The Lion, The Witch and the Wardrobe is everyone's favorite.) It is written for children but has enough depth that adults can enjoy the book. I am currently reading it in French to improve my fluency. I would also suggest a history series written for children (but again, not a childish set of books) entitled Story of the World (four volumes) by Susan Wise Bauer. Several similar well written (and older, beautiful use of the language) children's books are available at the Baldwin Project: http://www.mainlesson.com/displaybooksbytitle.php. My favorite writer there is HE Marshall (An Island Story is about the history of England). I suggest children's books (if they are well written and interesting) because I find it best to start out in a comfortably easy reading level and move up. If you are ready for more, and I noticed you mentioned science fiction, American writers Orson Scott Card (Ender's Game) is one of my husband's favorites and my adult/teen sons like it also. Sharon Shinn's Archangel series is a favorite of all the adults in my house. If you want a historical novel, Philippa Gregory has written a long list of amazing books that are average-native-level reading. Enjoy!

Finnegans Wake by James Joyce

If you are happy to read children's books and would like something funny, then most of Roald Dahl's books would be a good place to start. I particularly like "The Twits." Also worth considering are the books of our former Poet Laureate, John Masefield, who wrote "The Box of Delights," and "The Midnight Folk," which are aimed at children. A book which isn't aimed at children but which is very well written is George Orwell's "Animal Farm," which is a famous allegory of communism. It uses an apparently simple story about animals on a farmyard to tell a tale about communism from the view point of the author.