Behind the Green Door by Mildred A. Wirt Benson

Penny Parker is a teen-aged sleuth and amateur reporter with an uncanny knack for uncovering and solving unusual, sometimes bizarre mysteries. The only daughter of widower Anthony Parker, publisher of the "Riverview Star," Penny has been raised to be self-sufficient, outspoken, innovative, and extraordinarily tenacious. Her cheerful, chatty manner belies a shrewd and keenly observant mind. Penny was the creation of Mildred A. Wirt, who was also the author of the original Nancy Drew series (under the pseudonym Carolyn Keene). Wirt became frustrated when she was pushed to "tone down" Nancy Drew and make her less independent and daring. With Penny Parker, Wirt had a freer hand and received full credit. Wirt once said, " 'I always thought Penny Parker was a better Nancy Drew than Nancy is." In BEHIND THE GREEN DOOR, the Parkers' long-planned trip to the Pine Top ski resort for the Christmas holiday is upset when the "Riverview Star" is sued for libel. Publisher Anthony Parker remains behind to deal with the crisis while Penny is sent to Pine Top. There, she encounters multiple mysteries, including an elderly recluse who keeps his granddaughter a prisoner in his remote cabin and the unexplained presence of a reporter from a rival newspaper. Worse, the man suing the "Star" also shows up in Pine Top. Penny's nose for news combined with her penchant for dare-devil skiing soon land her in the middle of a dangerous tangle she must unravel to save both the "Star" and the ski resort.