[My legitimate hyperlinks in bold green]
Even casual readers, browsing pages of David Kahn's The Codebreakers: The Story of Secret Writing will encounter an intriguing illustration on page 830.
David Kahn Scrambles His Signature in The Codebreakers (p. 830)
See my research presentation that demonstrates how the Zodiac – Arthur Leigh Allen – likely emulated this example by scrambling his own name in his "Exorcist" letter to the SF Chronicle:
PRESENTATION: Zodiac Exorcist Letter - Glyphs Puzzle Solution
In 2007 while preparing an article about David Fincher's Zodiac film release, an SF Chronicle editorial assistant discovered what is possibly the last known correspondence from the killer. The 1990 American Greetings Christmas card, December postmarked from Eureka, CA, is a "Secret Pal" style evocative of the Zodiac's 1970 Halloween card to Paul Avery.
The red envelope had no return address; the card was unsigned and unmarked, but curiously enclosed a photocopy image of two keys on a chain.
Two Symbolic Cryptology Keys in Kahn's The Codebreakers (p. 134)
The rest of the caption:
Behind Trithemius, another person—either another monk or the publisher—extends towards Maximilian two keys to the book, these symbolizing Maximilian's spiritual authority and temporal power. In the background Trithemius' chaplain, a young monk, holds his abbot's crozier. At bottom, Trithemius reclines with a fruit-laden branch representing the motto "Ye shall judge the tree by its fruits" and implying that Trithemius' many works make him worthy of acclaim.
By 1990, the best-selling book, Zodiac had been out for four years and readers – including the killer – would have recognized the significance of The Codebreakers to Robert Graysmith's investigation. Specifically, his study of the Zodiac's cipher systems had led Graysmith to infer:
The Zodiac must have had a copy of this book.
See my research discoveries of additional compelling evidence that links the Zodiac Killer case to The Codebreakers:
PRESENTATION: Zodiac Halloween Card Mystery Symbol - Decoded
— Robert Peter Ackerman