We need a bit more information on what level you want. The GMAT is probably going to have rather advanced vocabulary. (though if you speak a romance language, that can help a lot.)
Easy: Harry Potter. Somewhat advanced vocabulary for a children's series.
Medium: The Dan Brown novels: Angels and Demons, Da Vinci Code. A little tougher would be anything by Orson Scott Card: Ender's Game, Speaker for the Dead. Neil Gaiman: American Gods, Stardust, Neverwhere. Some graphic novels can be fun: Sin City, Sandman. Douglas Adams: Hitchhiker's Guide to the Galaxy. Kurt Vonnegut: Slaughterhouse 5, Cat's Cradle. Ray Bradbury: Fahrenheit 451. George Orwell: Animal Farm, 1984.
Hard: For classic English, anything by Dickens. Neal Stephenson: Cryptonomicon, Snow Crash (lots of made up slang, though). Tolkien: The Hobbit, Lord of the Rings.
Let us know what level you want, and what kind of genre you like.