"Codex", by Lev Grossman...

"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).