The Way I Found Her By Rose Tremain

This is the summer that Lewis Little, precocious thirteen-year-old, is spending in Paris with his beautiful mother, Alice, who is translating the latest medieval romance by Valentina Gavrilovich, the bestselling and exotic Russian émigré. This is the summer that the bewitching Valentina beckons from her sofa, and Lewis discovers an exquisite new world filled with passion and intrigue, set against the alluring backdrop of Paris. 
But when Valentina disappears and Lewis takes it upon himself to find her, wondrous secrets suddenly turn sinister. This is the summer that Lewis, caught in a bizarre and dangerous romance, is about to face head-on the perilous force that transforms children into adults.