Anna, Where Are You? by Patricia Wentworth

There is no doubt in Thomasina Elliott's mind that her oldest schoolfriend is a creature of habit. So when, after three years of regular correspondence, Anna's letters suddenly stop, Thomasina becomes concerned. And even more so when it transpires that her friend has disappeared without trace, in extremely odd circumstances. Fortunately, however, the indomitable Miss Silver is on hand to investigate.
Binding: Paperback / softback