Jeffc,
When it comes to establishing purpose, it does not have to encompass the entire book. Rather, define a purpose for each chapter.
In other words, gear your purpose towards the parts and not the whole.
There are many great threads on the discussion of 'purpose' ...
Purpose is your destination.
Purpose is your desire.
Purpose is your goal.
Purpose is your outcome.
Purpose is your reason.
Purpose is many things, the nice thing is it is totally unique as it is derived by none other than yourself.
