Conversation so far:
What is Marie Stopes' Married Love?
Married Love or Love in Marriage is a book by British academic Marie Stopes.
What was Marie Stopes' marriage book called?
Married Love or Love in Marriage is the title of the book by British academic Marie Stopes.
How many copies did Marie Stopes' Married Love or Love in Marriage book sell?
Married Love or Love in Marriage went through 19 editions and sales of almost 750,000 copies by 1931.

Question: what was the follow up?
Self-contained rewrite: What was the follow up to Marie Stopes' book Married Love or Love in Marriage?