I have absolutely no idea how far I am in Thou Shell of Death, because I am reading it as part of a kindle anthology that includes two Campion shorts, this full length mystery, and a second full-length mystery by Michael Innes called There Came Both Mist and Snow. This is a Kindle Unlimited offering which I grabbed because it was listed as a Margery Allingham book. I'd previously read both of her short stories - On Christmas Day in the Morning and A Word in Season - so that turned out to be a bit of a bust.
However, Nicholas Blake and Thou Shell of Death pop up in Chapter 24 of TSoCC, which is a chapter I haven't yet filled. So, Christmas Mystery + free + Detection Bingo square = must read right now.
It's a classic impossible crime - one set of tracks in the snow to the deceased, none leaving the place where he was murdered. Nigel Strangeways, the amateur detective, is clever and likeable and the writing has a really nice flow to it - better than a lot of the Golden Age mystery writers.
So far, this is shaping up to be a good start to 2019!
I'm not sure if I will read the Innes book - he has another one - Death at the President's Lodging - which is a featured book for Chapter 11, Education, Education, Education. I've actually checked off that square, but I love mysteries set in British schools, and it is also available free from the KU library, so that might be something to read soon.