Алексей Зуев

var book = {
  author__1: {
    name: 'John',
    surname: 'Smith'
  },
  author__2: {
    name: 'John',
    surname: 'Smith'
  }
};
var max = Object.keys(book).length;

for (var i = 1; i <= max; i++) {
  console.log(book['author__' + i].name);
}