Any book by E.B. White (Charlotte's Web, Stuart Little, The Trumpet of the Swan); Animal Farm by George Orwell; The Swiss Family Robinson by Johan David Weiss; Of Mice and Men by John Steinbeck. These are easy-to-read classics that can be enjoyed by readers of any age.
Links to more suggestions:
http://www.goodreads.com/list/show/36278.Books_for_ESL_Readers_in_Middle_School
http://www.goodreads.com/list/show/75400.Great_Books_for_Cambridge_ESL_Students
Don't worry about the age categories. I'm a middle-aged adult with excellent reading skills, yet many of my favorite books were written for teens. Happy reading!