Thanks for the compliment on my book.
The DNA issue with respect to the Z letters has nothing to do with storage, if we can assume that the 1974 and 1978 letters individually, and all the other Z letters collectively were stored under about the same conditions. Had the saliva, cells and DNA on the 1969-1971 letters "degraded," you'd expect to see at least some degradation of the DNA on the 1974 and 1978 letters, since the amount of time that passed before DNA analysis became available is not much different for (especially) the 1971 and 1974 letters (i.e., three years) compared to the amount of time that passed after 1974 until DNA technology became available (i.e., the difference between 1971 and, say, 1998 is 27 years vs. 24 years between 1974 and 1998). This amount of time would presumably have given the 1974 DNA plenty of time to degrade if that were the issue. Alan Keel said that the DNA on both the 1974 and 1978 letters is easily extractable, such as one would find in a letter that was licked by the sender.
I was told that the Zodiac letters are segregated by the lab into two groups: the "true" Zodiac letters, which are characterized by very few cells and only small amount of saliva, vs. the forged 1974 and 1978 letters, where there is an abundance of saliva, many cells and DNA that is easily extracted and matched between them.
We can't presume that The Exorcist letter is the other forgery but it is my best bet based on what I know from the 2000 chart of letters that were tested for DNA and the accusations against Toschi from 1978.
This is all in my book (except the degradation discussion).
Mike