Behind the Green Door by Mildred A. Wirt (Penny Parker #4)

BEHIND THE GREEN DOOR is #4 in the Penny Parker Series by Mildred Wirt Benson.

Penny visits a ski resort and stumbles upon several mysteries. Unknown enemies are trying to force the resort to close, and during her investigations, Penny discovers a mysterious green door in the resort that leads to a room that can only be entered by invitation.