Recall that Stan has a Secret hide-away behind a vending machine. Recall again in ep 5 he left the store to handle something. This is pure speculation, but the Mystery Shack might just be a huge ruse to fool people, think Mild Mannered Clark Kent is Supes, or Billionaire Playboy spoiled rich-kid Bruce Wayne is the ever serious ass-kicking Batman. I doubt he wrote the books, but he may know the author, or just found the first one as a child, leading to the creation of the Mystery Shack. As for the six fingers, it may be symbolism to the 6th sense or something, the palm itself could be a symbol for something else. We can never know until we see more of the show.