“Codex” (2005) , another Voynich-ish thriller to add to the Big Fat List, is by New-York-based writer Lev Grossman who you might know as the author of the “When Words Fail” article (a nice introduction to the Voynich Manuscript) in April 1999’s Lingua Franca I have favourably cited here several times.
In his novel, an investment banker gets roped in by a wealthy couple to track down a medieval travel narrative, which may or may not be a fraud. Lev’s website says (of his own book) that “It’s also an unusual love story, as well as a love letter to the mysteries and wonders of the Book, the death of which has been wildly exaggerated“: sounds plausible to me. 
Oh, and it genuinely does appear to be an international bestseller, as evidenced by the 25 copies of it in my local libraries (where most interesting books don’t even merit a single copy, sadly).