Mental illness isn't something that goes away or can be convinced away because you want or need it to. This book does a heartshattering, brutal and important job of shining a light on mental illness and how it can feel, manifest and present. Obviously, everyone is individual and that is also talked about but it does an incredible job of showing both sides of a coin. For Theo it's about finding out how to support and love James. James needs the support and love of the people around him, more than anything but its about finding out how to ask for it, how to have it. 
It's not an easy read or without serious conversations and you can't read this if you aren't in the head space for it. But I think it's a really good read.