Tommy and Tuppence are old! But they can still solve mysteries -- even if those mysteries involve people who died long ago.
International intrigue... and puttering around in gardens. This book can be a little slow, but it has a great premise. Tuppence starts out by finding a secret message in a book... written by a young boy who died ages ago, and suspected something terrible.
Tommy and Tuppence are now in their 70s but still manage to be on the fringe of secret service work. Cant Darby and Joan even buy a country fixer-upper without fiding themselves, by accident, amidst a pre-WW I espionage case that was never solved? Although not essential to the story, you may want to consider reading (or rereading) N or M? first as there are many throwbacks to their WW II secret service role. If you wish, go all the way back to the beginning with The Secret Adversary. This is one of Aunties better intrigues as she builds clue upon clue, although you may not find a satisfactory resolution at the end.