For example, "Indiana Jones and the Philosopher's Stone" (Max McCoy, Bantam Books, 1995) is based entirely around the Voynich Manuscript: set in March 1933, a mad scholar called Sarducci has stolen the VMs (which is actually a map), and so Indy chases him through Mussolini's fascist Italy all the way to an amazing alchemical crypt in the desert... From what I've read of this book, it actually seems to be a pleasantly pulpy read, very much in tune with the actual VMs, and with a refreshing lack of power-mad Jesuit priests. However, I should warn you that it will be re-released on 29th April 2008, presumably to try to ride the whole 2008 Voy-niche publishing wave. *sigh*
Another pair of VMs-themed books came to my attention via the Bellairsia blog, which is devoted to books by the writer John Anthony Bellairs. His most famous novel was "The Face in the Frost" (1969), a fantasy novel in which Prospero and Roger Bacon fight against a mysterious grimoire that sounds not at all dissimilar to the VMs. After Bellairs' death in 1991, his estate commissioned author Brad Strickland to complete and continue Bellairs' various series: and it is in one of these that protagonist Johnny Dixon faces "The Wrath of the Grinning Ghost" (1999), which features the VMs in a starring role.
Connections between J.R.R.Tolkien and the VMs have been suggested in the past. According to Voynich mailing list member Anthony, Tolkien did indeed own a copy of at least one page of the VMs, which may have played a small part in influencing his choice of the fantasy scripts in his books. As I recall, there were a number of people in Tolkien's Oxford circle that had an interest in early modern scientific manuscripts, so this does seem a perfectly sensible idea.
Many people have also wondered about the relationship between H.P.Lovecraft's Necronomicon and the Voynich Manuscript: Colin Wilson based a short story called "The Return of the Lloigor" around this, and returned to the theme at the end of his novel "The Philosopher's Stone".
All the same, this modest pile of Voynich fiction looks set to triple in height this year... interesting times!