There's a handful of books from Moonstone currently going free at Amazon UK. They're not entirely mysteries, but then again they don't fit 100% comfortably anywhere else, either.
Full credit to Mr Wilson on another forum for pointing out the last three.
The Avenger Chronicles
Out of tragedy, a hero is born! In the roaring heart of the crucible, steel is made. In the raging flame of personal tragedy, men are sometimes forged into something more than human. Wealthy and successful at an early age, Richard Benson was preparing to enjoy a long and happy life with his family when crime took away his wife and young daughter. Once he was just a man, but now he is a machine of vengeance dedicated to the extermination of all crime. A figure of ice and steel, but more pitiless than both, Benson has become a symbol to crooks and killers--a terrible, almost impersonal force, masking cold genius and a nearly supernatural power behind a face as white and still as a dead man's mask. Only pale eyes, like ice in a polar dawn, hint at what awaits criminals when they invoke the rage of millionaire adventurer Richard Benson - The Avenger!
Now, for the first time in over 30 years, the fearless/expressionless crime fighter; the man with the moldable face, the man with the shock white hair and the pale grey eyes, is back in action in a stunning collection of stories featuring all the action, adventure, and revenge Avenger fans have come to expect! From noir adventure and two-fisted action, to emotional tales of inner demons, join The Avenger for the E-ticket thrill ride of your life!
The Green Hornet Chronicles
With his faithful valet Kato, Britt Reid, daring young publisher, matches wits with the Underworld, risking his life so that criminals and racketeers may feel the weight of the law through the sting of the Green Hornet!
Moonstone is proud to present The Green Hornet Chronicles, the first anthology featuring all-new original crime fiction tales of the man who hunts the biggest game, public enemies that even the FBI can't reach!
It's the mid 1960s, the political climate is shaky, there's civil unrest, freedom and equality issues erupt everywhere from film to music to out in the streets. On police records the green Hornet is actually a wanted criminal, a master manipulator, a crime boss who has his fingers in every pie.
In reality, The Green Hornet is actually Britt Reid,m owner-publisher of the Daily Sentinel. His goal is to destroy crime from within by posing as a criminal himself.
Kolchak: The Night Stalker - The Lost World - CJ Henderson
After getting a serial killer to confess, Kolchak is offered an international assignment with massive coverage around the world. With fame and fortune finally within reach, Kolchak is ready to cover the story, when he's confronted by a mysterious monk who warns him that "the seventy-two must always be."
What this means is not explained. But, before he knows it, Kolchak’s dreams are invaded by unexplainable images that let him know every step he takes toward this story is bringing him closer to death.
The Lone Ranger: Vendetta - Howard Hopkins
The Masked Man in a brand-new adventure! From out of the past comes a mysterious killer systematically murdering anyone with a connection to the Masked Rider of the Plains former identity. When all signs point to Butch Cavendish, a man long dead, The Lone Ranger finds himself trapped in a deadly game of cat and mouse with the life of his faithful Indian companion hanging in the balance!