Pip and Posy and the Christmas Tree is my DS favourite at the moment, much more christmassy than the Snowy Day, so I'd suggest that one too if you like the others.
Younger books wise my son also really likes Spots Christmas and Dear Santa, but at nearly 4 we won't get much longer with those.
We love all the Shirley Hughes ones: Lucy and Tom's Christmas, Alfie's Christmas, Snow in the Garden a first book of Christmas, Angel Mae, My Naughty Little Sister and Father Christmas (this one only illustrated by Shirley Hughes). There is a new Dogger Christmas book out this year too.
I love Pick a Pine Tree by Patricia Toht.
Judith Kerr Mog the cat, there are two Christmas stories.
I saw Jan Brett books suggested on here lots of times and it didn't really appeal much from googling but I got one second hand and I'm glad I did. I think we have The Trouble with Trolls and another Christmas one. There are lots of Christmas themed ones and I'd recommend getting at least one for the collection.
Dr Seuss Grinch who stole Christmas.
Richard Scarry The Best Christmas Book Ever
Lynley Dodd Christmas Crackers
There is a new Jane Hissey Little Bear one just out too, that I've bought but not read yet.