He means he's WRITING a book.
The Big Bang Theory Season 11, Episode 12
Put the pen down, dear.
We both know you're not WRITING anything.
Game of Thrones Season 5, Episode 6
I've been WRITING
about my mother!
The Big Bang Theory Season 11, Episode 15
Yes. Are you WRITING a book?
The Big Bang Theory Season 4, Episode 22
for WRITING "Wake Me Up
Before You Go Go."
The Big Bang Theory Season 5, Episode 15