The book I read somewhere early 70’s, young black girl explores deserted mansion through a secret entrance, ends up solving mystery of the heiress that disappeared and Dad becomes ‘keeper’ of house/museum.
Stump the Bookseller
New format, same book stumper solving.
House of Dies Drear or the sequel Mystery of Drear House https://en.wikipedia.org/wiki/The_House_of_Dies_Drear ?
If the girl wasn’t black I would say the Velvet Room by Zilpha Keatley Snyder.
Perhaps one of Patricia Elmore’s Susannah books? Susannah and the Blue House; Susannah and the Poison Green Halloween; Susannah and the Purple Mongoose. Might be too late for the book you’re looking for, but just in case