A blog reader wrote seeking book identification help that I can't provide; maybe you recognize it?
"I originally read it in the UK back in the late 80s/early 90s and it involves time travel (via a magic door?) to a house during world war 2. The home was populated by two children (a boy and a girl) with their dog called Blackie and a housekeeper who looked after them. The boy used to step through a door in the modern era and end up back in this house where he couldn't leave the home and neither could the children (or they would cease to exist).
The main tension point of the story was where the dog escaped one night during heavy bombing and the children were frantic and desperate to get outside to save the dog leading to some chicanery by the time travel to remedy the situation."
It sounds like one I'd like, and I wasn't able to find it through google....
Update: a friend recognized it, and it's not a time travel book after all. It is Nicholas Fisk's "A Rag, A Bone And A Hank Of Hair." My friend described it thus: "It's about a
boy at the end of the 22nd century who is employed to help with an experiment
involving "reborns" - a family from the 1940s who have been cloned. He can only
interract with them in their house, because they aren't allowed to know that
they're not still in their own time."