In our small-town library, sometime between 1956 and 1959, I becamse fascinated by a large-format story book with darkly forbidding (Gothic!!) illustrations (maybe engravings, or duo-tone illustrations). I believe the story was about a girl and boy (sister and brother?) who were either lost or held captive by ___?? in a forest or a cave or some outlandish place. They were obliged (I think) to do manual labor of some kind — carrying something (ore?) or digging for something. I recall that it was a book my grandmother would not let me take out of the library on her card! I was only a beginning (but precocious) reader at the time (age 7, approximately), so cannot recall the title or the names of the characters, etc. But the words “forbidden” “hidden” “forest” “treasure” “lost” all seem likely terms to be associated with this book. I would LOVE to find it, read it, and solve this mystery before I die!!!