(CNN) -- To most people, the literary debate over who wrote the works of William Shakespeare would appear to be much ado about nothing. After all, the play's the thing, right? What does it matter who wrote it?

To James Shapiro, however, it matters a great deal.

The Columbia University professor and Shakespeare scholar spent 15 years working on his 2005 book, "A Year in the Life of William Shakespeare: 1599." The work exhaustively details a key year in the Bard's career, when he wrote "Henry V" and "Julius Caesar" and became the man thought of as history's greatest English-language dramatist.

And yet he couldn't convince the doubters, who believe that the name "William Shakespeare" is a front for the real author.

"I thought I did a damned good job showing that it could only have been Shakespeare who wrote the plays we attributed to him," he said. "And I naively thought, that will slow people down who think that Shakespeare didn't write Shakespeare. And they kind of stepped around it.

"And I thought, I have to stop and really address this."

The result is Shapiro's new book, "Contested Will." In it, Shapiro chronicles the history of the anti-Stratfordian movement, which has believed that any number of people -- the essayist Francis Bacon, the nobleman the Earl of Oxford, Walter Raleigh, Christopher Marlowe -- wrote the plays ascribed to the glovemaker's son from Stratford-upon-Avon, born 446 years ago. It's a theory that has attracted some famous minds -- including Mark Twain and Sigmund Freud -- and will soon be coming to the screen as Roland ("2012") Emmerich's latest film, "Anonymous."

To the anti-Stratfordians, Shakespeare -- who left behind, by modern standards, relatively little in the way of personal records -- was too unworldly, too unromantic (in his will, he famously left his widow his "second best bed"), too ordinary to have written some of the greatest plays and poems known to man. It's a theory Shapiro roundly rejects. He says that modern audiences are reading Shakespeare through modern sensibilities, believing that the author's work is autobiographical -- which was not the case in Shakespeare's day.

"We read today anachronistically -- we expect to find things in books that were written 400 years ago that people writing 400 years ago would not have put in those books," he says. "But the plays of Shakespeare and his contemporaries were not [autobiographical]. ... They made stuff up based on stories they read."

The distinction is important, he adds.

"Part of the authorship question is about trying to find whose life you can line up with the life of the author of the plays. Oxfordians will say, hey, our guy had three daughters and was captured by pirates. Your guy had two daughters and was never captured by pirates. Therefore, our guy has a greater likelihood to have written the plays. And that way madness lies, because then you end up with 50 or 60 contenders."

Naturally, Oxfordians disagree. Shapiro's book has been wounded in its Amazon rankings by reviewers who don't believe Shapiro's thesis and criticized by anti-Stratfordian websites.

"I do think there is an authorship question," says Michael Egan, a retired English professor who edits the Oxfordian, the journal of the Shakespeare Oxford Society. Though he considers himself a Stratfordian, he says he's "open-minded" about the issue and criticizes Shapiro for some of the arguments in "Contested Will."

"The case for Oxford derives from the fact that almost everything we know about Shakespeare of Stratford doesn't seem connectible to the author of the plays," he says. "It's that gap between what we could infer about the author, and what we know about Shakespeare of Stratford, which has raised the questions."

It's a battle that has, as Shapiro records, been filled with partisan rhetoric and bad blood since it began a little more than 200 years ago. Twain, for example, wrote a short book on the subject; his contemporary, Henry James, also questioned Shakespeare's authorship. Others have created elaborate codes or sought biographical parallels. The Stratfordians stand by their proof; the anti-Stratfordians fill in the gaps.

In that respect, the battle over Shakespeare has much in common with other disputes. Kathy Olmsted, a history professor at the University of California-Davis and the author of "Real Enemies: Conspiracy Theories and American Democracy," observes that, in American history, conspiracy theories have often arisen from a perceived lack of information.

"When people don't have that information or can't get it, they like to sort of speculate on what the real story is," she says. "People see those blank spots and they want to fill them in."

And in for a penny, in for a pound, she adds: "It becomes like a religion. People who believe in these theories really get invested in them, and they don't want to account for evidence that doesn't fit their thesis."

Which is why, for Shapiro, it's so important to establish that Shakespeare wrote Shakespeare.

"So much is at stake in Shakespeare. In the great game of capture the literary flag, this is it," he says. "If you can say something about Shakespeare, you can say something about how English literature and literature in general works."

He's particularly leery about Emmerich's film, which is in production and stars Vanessa Redgrave, Rhys Ifans and Derek Jacobi -- the latter a noted Shakespeare doubter.

"It's going to be a disaster movie for people who teach Shakespeare," he says. "In the great rock-paper-scissor of movie and book, movie beats book."

Still, he's hopeful that "Contested Will" will have an impact. "You can't win that battle. But I can wage it, at any rate."

Nevertheless, he's ready to put the Baconians, Oxfordians, Marlovians and other anti-Stratfordians behind him.

"I can't wait to get back to Shakespeare and writing a book. I actually get smarter wrestling with Shakespeare's words -- it's very thrilling," he says. "But spending five years thinking about people's fantasies did not make me smarter at all. ... Was it worth it? Probably. As long as the next book goes well."