There’s no mystery about what the biggest book of the summer will be: Dan Brown‘s “Inferno” is coming out on May 14, and his publisher won’t let you forget it.

Today is the 10th anniversary of Brown’s phenomenal blockbuster, “The Da Vinci Code.” Doubleday is celebrating by giving away free e-copies of the 2003 novel that launched him into publishing history.

As an extra bonus, the e-book also includes the prologue and first chapter of Brown’s forthcoming “Inferno.”

According to Doubleday, “The Da Vinci Code” is the fastest-selling adult hardcover of all time. Eighty-one million copies have been sold.

In a statement, Sonny Mehta, the chairman and editor- in-chief of the Knopf Doubleday Publishing Group, said, “The original publication of ‘The Da Vinci Code’ preceded the e-book era. What better way to celebrate the 10th anniversary of this modern classic than by making it available on a multiplicity of platforms and devices while simultaneously offering fans a riveting sample of Dan’s latest work.”

Brown will only make one appearance in the U.S. to promote “Inferno”: On May 15, he’ll speak at Lincoln Center. Bookstores and libraries across the country are invited to live-stream the event.

The free e-book of “The Da Vinci Code” is available only in the U.S. and Canada, through March 24, but the prologue and first chapter of “Inferno” appeared in the Daily Mail in Britain yesterday.

The Lost Symbol” (2009), Brown’s previous novel about the adventures of Harvard symbologist Robert Langdon, took place in Washington. Its first printing was 5 million copies. Expect “Inferno” to burn up new records.

Twitter @RonCharles