Aquabird wrote:
I could be wrong, but I think The Lost Staircase was written well before Jesanne and Lois reached the CS - the references to WW1 make it sound as though it was set in the 1920s, and EBD did tend to set her books in whatever year she was writing them. Possibly it was one of her earliest efforts that she shelved, then revisited years later and was able to get published as a 'link' title to the CS series.
I know this is an old thread, but I was rereading Lavender, and Jesanne and Lois's adventures are quite dateable (although this doesn't help with the question of when EBD actually wrote their story).
Early in the chapter of Lavender in which Jesanne and Lois are introduced, Jesanne talks about her experience with games at her school in New Zealand, explaining that that took place before the war, so she must have come to the Dragon House prior to September 1939.
However it's also possible to date it even earlier than that as, later in the same chapter, Jesanne says that Mr and Mrs Jennings (Mr Jennings is agent to Sir Ambrose) had twins in the June before the war broke out. The arrival of the twins is mentioned at the very end of The Lost Staircase, and Jesanne's arrival is in the November prior, meaning November 1938. From that it's also possible to go back and work out the births and deaths of many of the family mentioned.