John Green is a talented writer with the ability to put together a gripping story and well-drawn characters. Turtles All the Way Down emphasizes the theme of mental illness, and interviews with Green have focused on how the obsessive compulsive disorder of the main character, Aza, reflects his own OCD. But the novel is as much about the loss of a parent and about how great wealth alters one's life and makes it difficult to assess the motives of other people who are friendly. It is Davis Pickett who lives in great wealth, but his father Russell has gone missing. Aza goes to the same school as Davis, and she used to have a crush on him. Her best friend Daisy wants to set them up together. Things quickly get complicated.Of course, there is romance, and while 16 year old Aza wants to kiss Davis, she can't stop thinking about the impact the exchange of saliva will have on her body and it alarms her so much that she can't bear it. She gets very self-involved and has to see her psychiatrist regularly, and she has learned various techniques to get her emotions under control. She has also been prescribed medication, which she rarely takes. She lives with her mother, who teaches at her school, and they have a good relationship, but Aza often withholds a lot of information from her. It makes it more difficult for her mother to help her. During the novel, Aza's problems get more serious and her behavior becomes especially bizarre. But there were times when she was more able to cope with her feelings and she hopes to return to such a state in the future.Aza has a strong relationship with Daisy and they often hang out together a lot. Daisy is far more outgoing than Aza and talks a lot, but there are tensions in the friendship. Aza is mostly focused on her own problems and pays little attention to Daisy's life. Eventually, in a crucial scene, Daisy voices her resentment of Aza's self-obsession. Aza's mental struggles mean that she has little energy to follow the lives of her friends in a real way, and Daisy questions whether they have a real friendship. It's a difficult and important question. It gets resolved, to an extent, with the idea that despite her limitations, Aza still has a lot to offer in a friendship.What's strong about Turtles All the Way Down is not the description of the mental illness itself, which is pretty standard, although it does do a good job of making sense of strange thoughts and actions, and how Aza manages to undermine her own goals. The strength is more in the way that Green shows the impact of Aza's mental illness on her relationships -- with her mother, friend and boyfriend, and even with herself. There's a similar inspection of the effect of great wealth and an unloving father on the lives of Davis and his younger brother. While the material benefits of money are very clear, Green is good at showing that it can make friendship much more difficult. Aza's mother is very suspicious of Davis, expecting that he will just want to exploit her daughter, and she is wrong about that. Aza is much more ready to take Davis at face value, but the money causes plenty of problems. It is probably rarer for YA novels to inspect the problems that come with wealth than it is for them to address mental illness, so it is this theme that makes Turtles All the Way Down more unusual. I strongly recommend this book.