Writers decide to organise their thoughts into books for different reasons.  Some people seek to document travel experience, to research and live vicariously through someone else's life, to devise fiction or to share lessons learned though their own personal or work experience.  Whatever the writer's motivation, what counts is its intensity.  The energy generated in a person's soul is what convinces that person he or she has good reason to share a specific story.

Preparing to write a book is a mental process.  Somewhere in your mind, you will decide this is something you would like to do for yourself and for the benefit of other people.  You may undertake research, collect, sort and edit material.  You may simply begin to transcribe words and see where that journey into yourself unfolds. From the point when you determine you desire to express things, its up to you to take initiatives and figure out how and for what audience.

Writing is a  vehicle that enables you to rediscover who you are and what you're made of.  Your chosen words also become tools that teach you what you're not.   You experience things and describe them in ways which elucidate your relationship with yourself and the world.  How truthful you choose to be reflects your level of ignorance and awareness, how much detail you wish to reveal and how deeply you hope to know yourself. 

In essence, you boarded the roller coaster of life.  You define the experiences you perceive or conceive are possible.  You live a choose-your-own adventure.  You decide what is reality and fantasy.  You can choose to write about the life you lead, the life you would like to lead and evolve into that as part of your own learning process.  No experience has meaning until you decide to give it meaning.  Everything or nothing means anything you choose. Share what you will.