When you know you have at least two books inside you, which book should you write (and publish) first?

The main thing that you need to remember is that a published book is for your audience--not for you. Otherwise, you could just write it and keep it as a diary or journal.

You need to ask, "What's in it for them?"

Write the book for your audience first.

Give your audience the goods, teach them the lessons, get them what they need and then you can write another book letting your audience get to know you on a different level.