PHILIP'S GIRL by Lucy Ferriss (Schocken: $14.95). After indulging in several casual affairs, Philip Decker, a mildly famous, married philosophy professor, seduces Annie Redfield, a student whose thesis he must oversee. Decker is not the only middle-aged philosopher at the small California college to be distracted from his calling by such worldly thoughts. But Annie falls in love with Philip, and Philip becomes more dependent on his protege than he had planned. Philip's lesbian daughter, unaware of the affair, finds herself intuitively drawn to Annie, and becomes doubly bitter when she learns the truth. Once proud of her autonomy, Annie is increasingly enmeshed within a tangled web of friendships, loves and family ties. When her May-September dalliance is exposed by an angry friend, the web unravels, leaving Annie dangling in midair with a bad case of vertigo. Ferriss is deft at charting the subtle shifts in power between a mentor and his protege, but the novel is frustratingly meandering and opaque. Despite a stylish bit of opening prose, Ferriss never manages to convey what makes the characters so irresistible to one another. The inner life of Philip's girl, the object of such intense longing and desire, remains an unengaging mystery.