Mary Stewart: Airs Above the Ground [1991] paperback

Vanessa March, two years married and very much in love, is propelled to Vienna by a shocking discovery. In her charge is young Timothy Lacy, who also has urgent problems to solve. But what promises to be no more than a delicate personal mission turns out to involve the security forces of three countries, two dead men, a circus and its colourful personnel and the famous white stallions of Vienna.