I have read this book before, I just reread it and I will read it again in the future. It's that good about putting words to mental health, especially depression and anxiety. I would know ; I'm unfortunately a victim of both.
Highly recommend, whether you suffer from it, or whether you're just curious. It truly is a great read. 
And that's coming from someone who isn't "such a fan", let's put it that way, of self-help books. Believe me : this one's the exception.