A children's book authored by U.S. President Barack Obama hit bookstore shelves in the United States Tuesday. The book, written as a letter from the President to his daughter Malia and Sasha profiles 13 prominent Americans. The book was written before Obama became president in 2008. It's part of a three-volume deal Obama agreed to in 2004, just after he was elected to the U.S. Senate. Obama's two other books Dreams from My Father and The Audacity of Hope were best-sellers in the United States.The 31-page Of Thee I Sing is published by Knopf. Royalties from book sales will go to a scholarship fund for children of U.S. soldiers killed or disabled.