If I didn't already know, and someone told me that the book "Philly" was turned into a movie, I would have said something like, "Well that's gotta be one pervy movie." I might add, "And what the hell did they do about the crazy shit at the end." The answers are that Private Lessons is a pervy movie and they totally ignored the dark ending. Probably a good move. It's what makes the book worth reading and it kept the movie in the slick, sleazy environment that it needed to be in order to be a commercial success. It must be noted that the author of the book, Dan Greenburg, is credited on the film screenplay and must have, on some level, agreed to the changes made.
For the first part of the book, it is often nearly word-for-word and scene-for-scene what's in the movie. Philly is a young teen (14 in the book, 15 in the movie) who finds he has ridiculously easy access to the nudity and sexuality of his new nanny/housekeeper, Miss Mallow. His airline pilot father is rich and often away for long periods of time and his mother has been dead for a long time. A plot involving money and scamming Philly by Miss Mallow and the chauffeur develops between scenes of dopey teen wish fulfillment. In the book, the chauffeur is a guy Philly identifies as "Lester the Fruit" who gives off a queer and bad vibe to Philly. Miss Mallow is in her early 40s (Sylvia Kristel, as the character, was in her late 20s when this was filmed). The book is an easy and laid-back teen novel at first. Nothing great, but readable. Sort of "Are You There God, It's Me, Peeping Tom".
Suddenly and unexpectedly, it turns into something far more dark and emotionally fraught than what it seemed to be developing into. For one thing, in the book, Philly is a dumb kid. Not just young but also dumb. He has no real sexual experience and his ideas are immature even for a 14 year-old. The film takes predictable turns as it exploits the sexual moments in the book. But the book is what happens as a result of those encounters and the movie ignores them. That's where what is creepy and fascinating lies. The movie doesn't want, or seem to need, these complications. The original story is a novella at just over 150 pages, and won't take longer to read than the movie is to watch. Because Sylvia Kristel is so beautiful and the movie has some squirm-inducing scenes, I still recommend watching it for those who are into that sort of thing. (Full disclosure: I'm into that sort of thing) But the book "Philly" is where you will find the real interesting experiment in storytelling.