Expatriate author Meg Rosoff, a Bostonian turned Londonite, has established herself as a crossover novelist whose work appeals to both teens and adults. In her three novels to date, she addresses youthful alienation and passion with a level of philosophical sophistication that rises above the histrionic angst found in much young adult fiction.

Her first two books, "How I Live Now" (2004) and "Just in Case" (2006), were published for young adults, with the former winning the Michael L. Printz medal, the most prestigious American award for young adult literature. So it's a little puzzling that her new title, "What I Was" (209 pages, $23.95), has itself crossed over onto Viking's adult fiction list, especially because, in many ways, this resonant story about an unhappy boarding school student's fixation on a self-sufficient orphan living in a hut by the sea is Rosoff's most innocent and ultimately hopeful book yet.

The narrator H (for Hilary) is a 100-year-old man looking back to 1962, when he was 16 and a new student at St. Oswald's School for Boys, a dreary, soul-killing institution on the Suffolk coast. Periodically throughout the text, H inserts one of his rules for living ("Rule number one: Trust no one."), perhaps to counteract the seemingly arbitrary regimentation inherent in his school days. "Our world revolved around school rules, rules as mysterious and arcane as the murkier corners of the papal cabal. Bottom button of blazer open or not, left hand in pocket or not, diagonal or straight crossing of the courtyard, running or walking on the lawn, books in right hand or left, blue ink or black, cap tipped forward or back."

A further contrast to this hemmed-in, middle-class existence is Finn, whom H first meets while jogging along the beach during phy-ed class. From H's perspective, Finn is everything H is not: beautiful, graceful, rugged, more than capable of surviving on his own, which he's had to do ever since his grandmother's death. He's an archetype of children's literature, from "Pippi Longstocking" to "The Outsiders" -- the child existing beyond adult control.

In a promotional interview with her publisher, Rosoff talks about her fascination with the fluidity of gender identity and sexual feelings in adolescence. H's obsession with Finn may seem sexual and twisted to his classmates, who torment him once rumors begin to circulate about his covert off-campus expeditions; but Rosoff conveys that H doesn't necessarily want to possess Finn, he wants to be him. Or, as H relates in reference to their first meeting: "He looked impossibly familiar, like a fantasy version of myself, with the face I had always hoped would look back at me from a mirror." Thus, while the pair's encounters are sexually charged (at least in H's mind), physical contact doesn't move past the dizzying moment when Finn grasps hold of H to keep him from falling off the edge of a cliffside cave, where they lie together one idyllic afternoon, watching the birds and the waves.

Rosoff deepens the narrative by infusing it with ancient British history. Hardly an eager or successful student, H nonetheless becomes interested in the Dark Ages, partly to impress the diligently self-educated Finn and partly because remnants of the era dot the surrounding land and ocean. Finn takes H out in a boat one day to see a submerged 14th-century fort, and, instead of the childish visions of smooth gray walls and scalable towers that H imagines beforehand, he finds an ominous, barely visible structure that exemplifies man's impermanence and nature's power, a power that will someday -- sooner rather than later -- submerge Finn's home, as well.

To the end, Finn remains an inscrutable character. Even a provocative climactic revelation about him doesn't illuminate his thoughts or motivations. Yet, when it comes down to it, Finn's personality is beside the point. He is a model to H of a harsher yet simpler and, at its essence, more honest way of life. Being able to partake in it, albeit fleetingly is, for Hilary and for Rosoff's readers (of any age), a significant experience.

Christine Heppermann also reviews for Horn Book magazine. She lives in Minneapolis.