The best person to help you write your book

Here’s a question: What’s the most fundamental point about writing books? 

A book is written to be read. Lots of people write for the sheer pleasure of writing. But if you’re writing for yourself, you’re writing a journal, or a diary. Not a book. Your book need only be read by one other person but that other person makes it a book. 

Do you know who that person is?