Every night before he goes to sleep, the president of the United States reads 10 letters from the pile of 20,000 sent to him by Americans every day. Sometimes, he writes back. He's even, on occasion, included a check. "It's not something I should advertise, but it has happened," President Barack Obama told reporter Eli Saslow, author of the new book, "Ten Letters: The Stories Americans Tell Their President." Saslow has spent a year poring over the presidential mail, sent from Americans weathering the desperate economic climate. There's Natoma Canfield, the cleaning woman battling cancer who can't afford to pay...