Write a prequel or sequel
Imagine you are the writer of this book and it was a big success. Now the publisher of the book wants you to write a sequel (a follow up to the original story) to it. Or a a prequel (a story which happened before the story in your book).
After a while the publisher wants to have a summary of 200-250 words at the most. If you write less she has to fill the blank space and If you write more, she has no room in the magazine.
She also wants you to end with a cliffhanger to make the reader more
willing to buy the book.