Why is it so hilarious that Norma Major wrote a book?

I don't understand why Hillary laughs when it is mentioned that Norma Major wrote a book while John Major was prime minister.

Was it just hilarious that she took the opportunity to write about Chequers? Was it just that she hadn't taken up a cause (like Hillary on healthcare) or didn't have a vocation (like Cherie being a barrister)? Were they just making light of it because she was the wife of a Conservative prime minister?