Turtles All the Way Down is a book about a girl living with OCD. And while I don't think it's the best John Green book and I'm not sure it's the one you would like best, it is my favorite by far. I don't have OCD, but I am mentally ill. And I have never read a book which conveys how it is to live with mental illness as well as this one. Almost every page contains a highlight of something Aza says which I find deeply relatable. The book is almost devoid of plot, but that is a good thing. Because that's part of how it feels to live with mental illness and it gives a lot of space to depict Aza's inner monologue.
I can't praise this book enough.