The Imposter's Inheritance, Paperback/C. J. Archer

A sordid scandal rocks the Glass household and threatens to ruin Matt's family unless he can suppress it. But after word gets out, and a rare magical coronet is stolen, he and India find themselves scrambling to recover the heirloom and suppress the gossip before reputations are ruined. To make matters worse, someone is sending threatening letters to the magicians of London, stirring up trouble between the artless and magicians. Tensions are rising, but India and Matt choose not to investigate - until the author of the letters is assaulted. As if they don't have enough on their plate, Willie is jailed. The reason for her arrest is...complicated.