The interesting thing is the way the three men disagree in their interpretations, each going his own way. Take for example the one sign that is constantly being rehashed in all the "Grammar and Alphabet" writings, the well-known reed-sign, perhaps the most important and certainly the commonest of all hieroglyphic symbols. A special treatment of the reed-sign is tacked on at the end of each of the three copies. A comparison of the three texts is instructive.
Each of these is interpreting the same sign, with no sovereign master-mind to bring them to a unity of the faith. Cowdery and Phelps hear different sounds and come up with different meanings. And Joseph freely lets them go their way while he goes his, each under obligation to "study it out in your mind" before asking for revelation. This is something that anti-Mormon writers have wilfully misinterpreted from the first.
I just love that last line. Let's repeat that: yes, folks, anti-Mormons have willfully misinterpreted this from the first.
Basically, what Nibley is arguing is that the three documents interpret the same sign in completely different ways. In fact, the documents don't even agree on whether the symbol is some variant of Za Ki on-hish or whether it's some variant of Ah-bra-oam! How damning! Or something. The trouble, of course, is that this is complete bull. Below are images of the actual documents. While they are certainly sloppy and contain omissions, they do not interpret the same sign differently. In fact, there are actually two signs here: the first is Za Ki on-hish and the second is Ah-bra-oam. Nibley's argument hardly even makes sense, let alone does it accurately represent the documents! This is the great Mormon apologist? The most charitable explanation I can think of is that he wrote this essay while doing a line or smoking a doobie!

-Chris