Tim O'Brien, executive editor of The Huffington Post and Arianna Huffington's main deputy in the newsroom, is leaving the site to work on his next book.

Huffington made the announcement this morning in an email to staff obtained by Capital.

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O'Brien was one of several New York Times veterans to jump to The Huffington Post several years ago. His departure is no doubt significant, as he was seen as one of the driving forces behind HuffPost's ongoing transition to a muscular reporting outfit. He was also the architect of the site's iPad magazine, Huffington, which launched last year but failed to build up an audience large enough for it to remain a paid product.

O'Brien's reporters will be handed off to other editors, including Howard Fineman, Peter Goodman and Ryan Grim. He did not immediately respond to requests for comment.

But sources said the announcement caught the newsroom off guard.

"What does it say that his departure would be announced over email and catch everyone by surprise?," said one insider.

Another source told Capital that shortly after the email went out, Huffington convened a meeting with top editors, newsdesk editors and national reporters. She cited a company policy dictating that employees can't write books while working at HuffPost, said the source, who was at the meeting.

"The policy is that anybody starting a new book must either leave employment or take a sabbatical," a HuffPost spokesman confirmed, noting that reporter Radley Balko, for instance, took a sabbatical last summer to work on a book.

O'Brien has written several historical novels to date. Last September, Huffington threw a book party to celebrate the release of his most recent one, a fictional thriller about the assassination of Abraham Lincoln.

Huffington's memo is below:

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

Ed. Note: This item was updated from an earlier version to include additional reporting.