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Fair Play Whodunnit: The reader might not have all the solid evidence until after the fact, but does have all the clues that tipped Thatcher off.
Mystery Magnet: The Sloan Guaranty Trust, "the third largest bank in the world", gets involved with a business, and some person involved with it ends up dead.
Never Live It Down: In-universe example: His grandson received a puzzle box for Christmas; the two of them enjoyed spending the day trying to solve it. That was enough of puzzle boxes for Thatcher, but his relatives thought he was obsessed with them.
Nice to the Waiter: A variant — Thatcher thinks (in an early novel) that he gets good service because waiters recognize him as a powerful man. It's really because they recognize him as a good tipper.
Real Life Superpowers: Thatcher has intelligence, some political influence, and of course money. In his first novel he gets answers from a reluctant airline employee because he's on the airline's board of directors. note At least, he claims to be on the board. It's implied that he may have been fibbing.
Rich Idiot with No Day Job: Averted for Thatcher himself. He is a hard worker, and expects the same of his subordinates. Played straight for bank president Bradford Withers, who appears to see his job as a collection of social contacts. On the rare occasion when Withers takes an active role, something goes wrong.
The Summation: The books normally end between the murderer's arrest or suicide and his eventual trial, so this is how Thatcher gets the other characters (and the reader) up to date on just what was going on.