In the sense of "capable of being read," legible and readable are interchangeable: "His handwriting was frail but legible (or readable)." Readable has additional meanings of "interesting," "pleasurable," "fascinating": "This is the most readable novel I have come across in weeks." "Although our son's letter from camp is legible, it is not especially readable." Corresponding nouns are legibility and readability. See also illegible.
Dictionary of problem words and expressions. Harry Shaw. 1975.