The Huffington Post's executive editor Tim O'Brien is leaving the site to work on the second installment of his five-book series, a spokesman told TheWrap.

O'Brien, a former New York Times Sunday Business editor and 2009 Pulitzer Prize finalist, is working for Random House on a series of historical thrillers set shortly after the assassination of President Abraham Lincoln.

"Tim has a book contract he wants to fulfill, and he wants to be fully engaged in his book writing," HuffPost spokesman Rhoades Alderson told TheWrap.

It remains unclear whether O'Brien will be replaced, but Alderson said his duties will be distributed to the senior editorial team of Howard Fineman, Peter Goodman, Ryan Grim, Kate Palmer, John Montorio, Sam Stein and Amanda Terkel.

Arianna Huffington, the site's founder and editor-in-chief, announced O'Brien's departure in an email to the staff on Tuesday. She praised his work building and overseeing the site's team of reporters and launching the iPad-only Huffington magazine.

For the last two years, he has led the national reporting team, which won HuffPost the first Pulitzer for National Reporting [5] last year for a 10-part series on post-traumatic stress disorder, called "Beyond the Battlefield." It was the first time a Pulitzer has been awarded to an online-only publication.

"I'm truly grateful to him for helping us build such a robust and well-oiled original reporting infrastructure," she wrote in the memo, obtained by TheWrap. "We'll miss him and look forward to his visits anytime he needs a break from the setting of his next historical novel."

Capital New York's Joe Pompeo [6] reported that Huffington convened a meeting of top editors and national reporters after sending the email, which took some by surprise, and cited a company policy that employees can't write books while working at HuffPost.

Alderson did not immediately respond to questions from TheWrap about the policy.

Here is Huffington's full memo:

After two years overseeing our national reporting staff in New York, Tim O'Brien is leaving to work on the second installment of his five-book publishing deal. I'm truly grateful to him for helping us build such a robust and well-oiled original reporting infrastructure, including a crack team of news editors and copy editors, for helping launch the Huffington. magazine, and for editing David Wood’s series that won HuffPost a Pulitzer. In the next few days, Tim's reporters will transition to the editorial team of Howard Fineman, Peter Goodman, Ryan Grim, Kate Palmer, John Montorio, Sam Stein, and Amanda Terkel. We'll miss him and look forward to his visits anytime he needs a break from the setting of his next historical novel.

As part of our ongoing -- and growing -- commitment to original reporting, I’m very pleased to announce the return of Shahien Nasiripour, who was one of HuffPost's first (and most dogged) reporters, before moving to the Financial Times in August 2011. When he rejoins us next month, Shahien will be on the financial and regulatory beat, including Wall Street, Washington policy, and the global response to the financial crisis.

So please join me in wishing Tim all the best, and welcoming Shahien back to the HuffPost newsroom.

Arianna