I read 'Ordeal by Innocence' the other day, and since then several more Agatha Christie novels. I now regard the current BBC TV adaption of 'Ordeal' a ridiculous travesty, the subtlety and insight of the original jettisoned in favour of a lurid ego trip by the director.
It seems to me that Agatha Christie's greatest gift was understanding the psychology of families. For me, too, there is the fascination of period detail - the comfortable middle classes on holiday in hotels, or in their country houses. What they ate, how they dressed - it's escapist in a way, but it is social history.