Good on you for unveiling the man in the background! "The Secret Adversary" is fun if you don't take it too seriously -- ditto "Partners in Crime" (the second Tommy and Tuppence book), and I think Christie didn't mean these two to be taken too seriously. Unfortunately that's not true of books 3 and 5 in the series, and that's precisely why those two don't work for me. Book 4 ("By the Pricking of My Thumbs") is essentially a retiring-age Tuppence going on a sleuthing expedition that isn't all too different from Miss Marple's and Poirot's, and I rather like it, even though you can tell it's one of Christie's later books, written at a time when none of her series were up to their previous standard any longer.