I can only assume that with the TV show a lot of people have rushed out and bought the book.
It really is available (normally) in the black covers and, indeed, Waterstones do normally carry it.
This is what it looks like:
You may have to be patient until they get more stock in. I even looked on the Transworld site (they publish Terry's books
) and they didn't have it either.
“Men never commit evil so fully and joyfully as when they do it for religious convictions.” – Blaise Pascal