Write a book when the following are true:
• You’re ready to move from writer to author.
• You have an audience you’ve built.
• Your book solves a problem for a certain person.
• You can distill that problem you solve into one sentence.
• Whatever you write…you write in a way others can recognize as you.
• You’re willing to be vulnerable.
Ann Handley expands on these concepts in her excellent newsletter.