Patricia Wentworth 
Anna, Where Are You? [EPUB ebook] 

There is nothing more deadly than a friend betrayed.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.