Films and TV series based on novels exist in parallel universes. What happens on the page often is vastly different from the action on the screen. That’s certainly been the case with Jeff Lindsay’s novels about Dexter Morgan — a blood splatter analyst for the Miami Police Department and avenging serial killer — and the Showtime series that stars Michael C. Hall.

As intriguing as Lindsay’s novels have been, they too often spin into fantasy, evoking the murky supernatural in the plot as Dexter preys only on murderers far more evil than he could be. The Emmy-winning series, which just launched its sixth season, acknowledges that reality is the scariest place of all.

Whether it’s Hall’s brilliant performance or Lindsay’s version, Dexter is, at heart, almost an innocent with fears and anxieties. He has no illusions about what he is as he struggles to cope with family life, his job, his lack of emotions — and how to channel his need to kill.

Lindsay’s sixth novel, “Double Dexter,” takes a tremendous leap forward as the author utilizes believable storytelling and large doses of dark humor. No otherworldly creatures or South Beach cannibals mar “Double Dexter” as the ethical killer finds he is being stalked by another person.

Dexter has just finished dispatching a serial child killer when he discovers he’s being watched. Dexter is sure his watcher will call the police. Instead, Dexter begins to get creepy emails that suggest he has a copy cat. A series of cop killings pit Dexter in the middle of the police investigation when a bombastic detective accuses Dexter of murdering a colleague. Dexter does double duty — trying to prove his innocence at this false accusation and find the mysterious killer. But Dexter is shaken by an even more frightening assignment — the family vacation to Key West, which turns into a rousing finale.

“Double Dexter” is Lindsay’s strongest outing as he richly delves into Dexter’s very scarred psyche. Although this is a series about a serial killer, Lindsay reins in the violence. “Double Dexter” spins on Dexter’s internal monologue as he ruminates about his wife, newborn daughter, step-children and his brother and the Miami housing market’s downturn. (Remember, the novels are different from the series).

Dexter continues to be one of the genre’s most unusual heroes.