Feel the fear and do it anyway was written for a wide demographic of readers, who don't have to be mentally unwell, just fearful of fixing their lives or improving it. The book is filled with realistic, doable daily tasks which set your emotions and mindset for the day. The tasks also have room to be adapted for you. Since they are adaptable and easy they aren't outdated.
The book is entirely an educated diary, that personal aspect to it is what made it sell. It's not a cash grab memoir that is written for mass production. It's written with true intent in helping one's self.