What are the most important ideas that you want the reader to take away from your books?
I write out of frustration, mostly. Usually it's because I've spent a lot of time finding (and compiling) information that should have been easy to get. If it was hard for me, then it stands to reason that it's hard for others, too.
Also, writing is the last step in learning. If you can write thoroughly enough that people you don't know can follow along, then you know you really deeply understand the topic.