Really, he should have dropped all his faux logical positivism guff (drearily moving the main character forward one atom at a time) and instead started from about page 190. Then, just three pages from the end, when the main character's mind is temporarily merged [a bit Mr Spock-y, but what the hey] with the mad God-like uber-priest K'tholo, Wilson could easily have punted the story off into an even higher state of Lovecraft emulation (but moved forward to the present day)... now that would have been a nice slice of occult horror to read. But he didn't. :-(
Including the Voynich Manuscript is a nice piece of intellectual decoupage on Wilson's part, but feels a bit like collateral damage from his high-speed drive-by scattershot blasts at culture, philosophy and history - Bruckner, Merleau-Ponty, George Bernard Shaw, Plato, de Maupassant, the Popol Vuh, etc - which fill most of the book.
Still, if you fancy reading a Lovecraftian short story disguised as a novel, you shouldn't be too disappointed. *sigh*