Trying to figure out which book Z might have been familiar with is probably a dead end after all. I was just thinking there could’ve been a popular fad around that time that would have narrowed it down to one book.

Funnily enough, I re-watched Zodiac movie the other day, and in it Graysmith tells Toschi a number of years later how he went to a bunch of libraries around the Bay Area and discovered that in 2 of them several cryptography books were missing. Which was a surprise to Toschi. Which implies that it didn’t occur to SFPD to follow up on that obvious lead originally. And also it seems to imply that the books were stolen to hide the record of who borrowed them. Which seems kind of far-fetched, considering it would’ve been much easier to steal the little cards inside each book, and not the whole books. If it were me, I would’ve even stolen or simply mixed up a bunch of cards from many unrelated books, to hide the fact that cryptography books were targeted in the first place, and to make it look like a stupid prank. But most importantly, the library must be keeping a separate record of who borrowed each book besides those little cards in the books themselves. So did they search those records and couldn’t find anything after all? Makes me think that a lot of things that were painted as "circumstantial evidence" were not as strong as they appears in the movie/book. But I digress. 🙂