Let's go behind the scenes of The Hidden Staircase for a few interesting items I've found in the Stratemeyer Syndicate files at the NYPL and also from my original outline of this story...
The original outline describes the Turnbull twin sisters as "eccentric but loveable."
The outline refers to Carson Drew several times as "daddy."
When Edward sent Mildred the outline for The Hidden Staircase in 1929, he included the outline for The Secret of the Old Clock for reference.
Edward wrote Mildred in a November 8, 1929 letter, "I trust that you will go a the new story immediately and try to carry it through in the style as the latter chapters of the story just received. Make the characters as vital as possible and connect the incidents as plausibly as possible and give it the intenseness of the latter part of the first story. Then I am sure we shall have what we want."
When he received the manuscript, he wrote Mildred on December 11, 1929 to say "I have received the manuscript of 'The Hidden Staircase' and read it with much satisfaction. It seems to me it ought to interest ay girl who likes mystery stories. I shall make only a few changes and those of small importance."
When writing Hidden Staircase, Mildred departed from the outline in one instance and wrote Edward about it. As the setting was the middle west, she changed the reference to The Mansion being from Revolutionary times to the Civil War to be historically accurate.