What happens next?

How to write a book with another person
Decide who you're going to write the book with.
It should be someone you get along well with, someone who likes to write, and someone who has a similar writing style to yours. Pick somebody that is open to new ideas and isn't bossy.
You need a person you can trust and who you'll want to share your ideas with. Find someone you can discuss ideas with easily and who values your opinion.