Mystery:Who Ghostwrote Obama's Books?

Barack Obama is not an accomplished writer. In fact, he's not much of a writer at all, having never published a single article under his name despite serving as president of a law journal and on the faculty for years at a law school. It took him years and a leave of absence from his job before one of his books was published late under his name.

So the mystery is this: who ghostwrote the books that Obama published under his name?