I'm not sure whether there is a book about it but there are links between diabetes and depression that are discussed in online articles. Part of it is just the day to day hassle of it all that gets people down. Part of it is biological/chemical. Poor control can often lead to feeling physically exhausted all the time, and a feeling of failure in many people. Good control often means more energy and you feel like you can be proud of something. Have a look online, there's loads of research that's been done.
Hope you're ok