The Midsummer Crown by Kate Sedley

The new Roger the Chapman mystery - In the sultry midsummer of 1483, as Richard of Gloucester begins his bid for the English crown, Roger returns home to Bristol, glad to be out of the capital. But almost immediately, the Duke recalls him to London to investigate the mysterious disappearance of a young boy whose tutor has been found murdered, apparently in a locked room. It is an investigation which has as its background an ancient British legend, and which will imperil Roger's life as never before.