The quote is from an ACD Holmes story, The Red Circle.
The ABC Murders was cancelled as an adventure game and only developed as a casual hidden object. They aren't planning any more Agatha Christie adventures, unfortunately. I enjoyed them too.
As for new Sherlock games, there's a puzzle/adventure coming out on Nintendo 3DS, called The Frozen City. It isn't being transported across to PC, though. The follow-up to Testament of Sherlock Holmes has just entered development, but it'll be a long while before that's anywhere near ready.