Madame Serpent (Catherine de Medici #1) by Jean Plaidy

Sullen-eyed and broken-hearted, fourteen-year-old Catherine de' Medici arrives in Marseilles to marry Henry of Orleans, second son of the King of France. On the promise of a dowry fit for a king, Catherine has left her true love in Italy, forced into trading her future for a stake in the French crown.

Amid the glittering fêtes and banquets of the most immoral court in sixteenth-century Europe, the reluctant bride becomes a passionate but unwanted wife. Humiliated and unloved, Catherine spies on Henry and his lover, the infamous Diane de Poitiers. And, tortured by what she sees, Catherine becomes dangerously occupied by a ruthless ambition destined to make her the most despised woman in France: the dream that one day the French crown will be worn be a Medici heir...