On January 15, Barack Obama signed off on an advance for a children's version of his "Dreams From My Father" book. That was less than a week before he took the oath of office: link.

Analysts say there don't appear to be any rules that would bar such transactions after a president takes office, but it's unclear whether an incoming or sitting president has ever signed a book deal upon entering the White House.

"I don't recall any sitting president entering into a book deal," said campaign finance lawyer Jan Baran, former general counsel to the Republican National Committee. "They all have historically done that after they leave office.

The half a million comes on top of millions that he's received in royalties from both of his current books.