As the book unfolds, the author will analyze the communications and reach various conclusions based on their content, claiming that the Zodiac Killer had a fascination or interest in the theater, pointing out the letters that featured The Mikado. Then present you with information that connects their suspect to the theatre. They will use the mention of "radians" in the July 26th 1970 letter to point out the Zodiac Killer was an intelligent person, who could have known about radians through his profession - but only if their suspect had a profession that would make this claim possible. They may point out the cryptograms as evidence for their suspect, if their suspect worked with computers, possessed any code breaking books, or had a background in mathematics. The author will carefully select elements of the communications that seem to bolster the argument for their suspect, while conveniently ignoring the elements that seem to go nowhere. This isn't a criticism - it's the obvious reality of selling a story - and the flawed approach of viewing the case through the lens of a suspect.
But if the author has concluded that the Zodiac Killer was serving us a deception by deliberately misspelling words in a communication, then why do they readily accept the remaining content of the communication to promote their suspect. The Zodiac Killer could have easily copied any text from any theater production, to deceive us into believing he had theatrical leanings. He could have read about radians in any magazine article or book, and simply incorporated them into his July 26th 1970 letter. If he can create deception in one element of his communications, he can incorporate deception throughout. Why do we readily accept he could have deliberately misspelled certain words to sell us a lie, yet not draw the same conclusion for everything else he presented? Therefore, when an author argues for a particular suspect based on the content of the Zodiac communications, they may be arguing for somebody the polar opposite of who we are really looking for. The Zodiac Killer may have offered us plenty of material with which to dissect and analyze the type of person we should be looking for, but the content he mailed to the newspapers was chosen by a killer many believe to be a liar. Running around town looking for people who played Ko-Ko The Lord High Executioner in school, may be just what the killer intended. While you're looking around one corner, the murderer appears from the other. If the author accepts the premise of a killer who used misdirection when he misspelled certain words, then they are building a house of cards when using the rest of the communications as an argument to support their suspect. In reality, their presentation is arguing against itself.