Answer: THE best reason to write is for yourself first so you got a good basis already.
The most important thing, if this is something you really want to commit to, is just to write it. Take an hour each day and write it; get it all out. You can always worry about structure and editing later. The hardest part will be getting it all wrote down. Whether it's good or not (whether you think it's good or not) doesn't matter.
It would help for you to make a quick outline of everything you want to include in your book. It doesn't have to be anything formal, just something that you can use to keep the pages flowing and you can add or take away stuff as you see fit as you go along.
But it can't be stressed enough that if you want to write it, then you need to just sit down and write. Clear everything from your screen (that includes Solitaire, too) and start typing. Even if you have no clue in your head what to write, just sit down and start typing. You'll be surprised at how much comes out and how quickly the ideas start coming when you do this.