Make an outline, it will help. An outline will allow you to jump around and work on those areas that interest you at that moment.
 
If you go off on a tangent, well... tangents aren't really tangents. Allow your writing to go where it may. You'll fit it in later...
 
Or you won't.
 
Let it go, don't outline in a very strict way. Let your thoughts wander and fit them back into the book later.
 
It doesn't matter whether you want to write an e-book, paperback, or an audio book; James recommends you do all three. You'll look much more professional.