Showing A Letter of Mary by Laurie R. King

 1923. Mary Russell Holmes and her husband, the retired Sherlock Holmes, are enjoying the summer together on their Sussex estate when they are visited by an old friend, Miss Dorothy Ruskin, an archeologist just returned from Palestine. She leaves in their protection an ancient manuscript which seems to hint at the possibility that Mary Magdalene was an apostle--an artifact certain to stir up a storm of biblical proportions in the Christian establishment. When Ruskin is suddenly killed in a tragic accident, Russell and Holmes find themselves on the trail of a fiendishly clever murderer.