Board Thread:Questions and Answers/@comment-44975735-20200305170443/@comment-44975735-20200525132204

Just a thought. If Secrets of the magic makers (that's how the book is called) was the book where Iduna and Agnarr collected their information about magic, then the part about trolls healing people and the map must also be in it. This means that the secret room may have appeared in Frozen when Agnarr was looking for the map to the Valley of the living rock.

edit: The secret room is not visible in the movie. Another possibility is that that all this and all other similar books were moved when Anna started to read a lot to make sure that she doesn't learn about magic.