I think there were 80 books. Some of these are novels and others are collections of short stories. But it's complicated by the fact that some of the books have been published at a later date with a different title. There are also some books which were published in the USA but not in the UK, although these seem to be mainly short story collections that are available in other forms in the UK. As well as this, she also published 6 novels under the name of Mary Westmacott and 15 plays. Finally, there was also a book that she co-wrote with various other people such as GK Chesterton and Dorothy L Sayers among others.