Jennings took off his sunglasses and polished them on a
spotless white handkerchief before replacing them.
'I guess Walinski's read some of the same files I have.
UNIT, Torchwood, Operation Yellow Book -the real deal,
not the sanitised cover-up stuff they put out under Freedom
of Information.'
'UNIT?' the Doctor said. 'You know who I am, then?'
I know what UNIT and Torchwood are, but does anyone know what Operation Yellow Book is referring to?
BTW, if you haven't read the book yet, (or if you borrowed the book to read it, and want to look at the quote in context) a free online PDF version is
here.