
He means he's WRITING a book.
The Big Bang Theory Season 11, Episode 12

is WRITING itself.
The Big Bang Theory Season 5, Episode 14

Hey, have you finished WRITING
your profile yet?
The Big Bang Theory Season 7, Episode 4

I remember sitting in your
bedroom and WRITING the program.
The Big Bang Theory Season 11, Episode 9

and if there's WRITING on it,
make sure it's not all caps.
The Big Bang Theory Season 9, Episode 17