What's an author? Duh. The person who wrote the book you're reading, obviously. Pretty simple, right?
Well, yes and no.
The idea of authorship has changed over time, and is shifting still. In early periods, authors like Big Willy Shakespeare wrote under a system of patronage; that is, royalty and aristocrats supported writers and artists financially, but they had some say in what those guys were writing.
Modern authors, on the other hand, get their dough from the literary marketplace. Instead of patrons, authors have to please those hoards of hungry readers, all clamoring for the Bestsellers shelf at Barnes & Noble. Plus, with the emergence of collectively written Internet sites like Wikipedia our ideas about authorship are changing even now.
Then there's the matter of what the author means to say.The relevance of an author's intentions have long been debated in literary criticism and theory. The New Critics rejected the idea that an author's intentions had any impact on a text's meaning. French critic Roland Barthes' famous 1967 essay "The Death of the Author" declared that authors, their intentions, and their biographies no longer matter when interpreting texts.
Take that, Twain. No one cares what you think.