That would really depend on the learner's level of English proficiency. Another factor would be the person's age, which would determine what type of subject matter might be interesting or appropriate. Generally speaking, these are some very well-known novels that are popular with younger readers which makes them relatively easy reads.
Older classics for younger readers: "Charlotte's Web" by E.B. White, "Mrs. Frisby and the Rats of NIMH" by Robert C. O'Brien, "Where the Red Fern Grows" by Wilson Rawls, "The Wind in the Willows" by Kenneth Grahame, and "The Yearling" by Marjorie Kinnan Rawlings
Newer books for slightly older readers: "Holes" by Louis Sachar and "Hatchet" by Gary Paulsen
Classics for younger readers that might not be as easily digestible for foreigners: The works of Roald Dahl ("The BFG", "Charlie and the Chocolate Factory", "Fantastic Mr. Fox", "James and the Giant Peach", and "Matilda"), "Alice's Adventures in Wonderland" by Lewis Carroll and "The Chronicles of Narnia" by C.S. Lewis
Popular recent young adult books: The "Harry Potter" series, the "Percy Jackson" series, "The Hunger Games" series.
More mature fare but still fairly easy: "Animal Farm" by George Orwell, "Fahrenheit 451" by Ray Bradbury, "Lord of the Flies" by William Golding.