Where Does the Money Go? : Your Guided Tour to the Federal Budget Crisis by Scott Bittle,

This is one of those books that if you think about it too much, you'd hide your money under the mattress and buy some ammo for fear of the coming crash (caused by the impossibility of the US meeting its Social Security and Medicare obligations). But because the book is so goofy and flippant in tone (and here I must admit that I bought it mostly because of the cover) that it is easy not to take it too seriously. In any case, this is fun and interesting reading.