The Voynich Manuscript makes its appearance very early on (p.27, actually the first page of Chapter 1): McCoy manages to present its history very lightly and not bog the reader down in too many details. But as the book is set in 1933, there wasn't a whole UFO angle to cover (or other such modern confections). Instead, you get a little bit of Newbold, Bacon, alchemy, Major John M. Manly (!!!), John Dee, Kelly, the Shew Stone, and even a quick reference to Wilfrid Voynich in New York: basically, everything moves briskly along in the kind of proper screenplay-like way you'd hope from an Indy novel. Yes, there's even the occasional snake (for readers playing Indy buzzword bingo, I guess).
I'll admit it: I was charmed by the book. It's small (293 pocket-size pages), no larger than you'd imagine a Japanese commuter squeezing into a pocket, and reads so quickly that at some points (most notably in the end sequence past the oasis) I deliberately closed my eyes to slow the pace down so that I could properly picture the scene in my mind.
Historically, the book has a deliciously light touch throughout, in particular when Indy and his companion are improbably rescued by an elderly French couple called Nicholas and Peronelle (p.200) - and if you can't work out who they are by that stage in the story, you very possibly deserve to be shot.
I liked all the atlantici history and the Shelta Thari stuff (there's a Wikipedia page too) woven in: but note that when McCoy writes "Nus a dhabjan dhuilsa", he probably means "Nus a dhabjon dhuilsha" ['The blessing of God on you'], though I'd prefer not to pick a fight with a tinker / tinsmith as to which one is correct. Incidentally, my guess is that McCoy picked up the reference to Thari from Roger Zelazny's 10-book 'Amber' series.
Inevitably, there are some historical mistakes in the book (the VMs wasn't in Yale in 1933, I'm pretty sure that the British Museum had a positive rotograph of at least some of the VMs in 1929, etc), but frankly I couldn't care less. It's a delightful, frothy, whip-cracking romp through alchemical history, that I think should be required reading for any modern Voynich novelist.