The hidden market devouring Twilight novels.

Vampires are having their moment in, well, if not the sun, then certainly the Twilight. Author Stephenie Meyer's series of books about the romantic yearnings of an undead teen are the, uh, lifeblood of the book business these days. According to USA Today, one in every seven books sold in the United States in the first quarter of an otherwise dismal 2009 was one of the four Twilight stories. On Amazon.com, half of the top 10 is made up of Twilight. (Each of the four books holds a spot, and the collected series takes up another one.)

Readers can't get enough of the forbidden love affair between a human girl named Bella and her bloodsucking but good-hearted beau, Edward. He's emo! He's chivalrous! And glittery! (Meyer takes some liberty with horror-movie convention; instead of burning and shriveling up when sunlight hits them, vampires literally sparkle.) What more could an adolescent girl want in a fictional boyfriend?

The real-life plot twist here, though, is that it's not 'tween and teen girls who make up Twilight's ardent—and profitable—fan base. It's their mothers. According to publishing research firm Simba Information, the number of teen-oriented books on the New York Times' and USA Today's best-seller lists has risen in recent years. But National Endowment for the Arts research that tracks the reading habits of teens shows their reading is flat or falling. Simba analyst Michael Norris says the disconnect results from adults devouring ostensibly teen-targeted fare. For a real-time snapshot of this unlikely fan base, hit the Internet, where grown-up Twilight fans have staked their claim. The biggest online fan site by far is TwilightMoms.com, which requires users to be (or at least to say they are) older than 25 and a mom to register as members. Founder Lisa Hansen says TwilightMoms has around 27,000 registered users. This is almost four times more than the next-largest fan site. All-ages TwilightLexicon.com, which is endorsed by Stephenie Meyer herself, has just fewer than 7,000 members.

Amazon's customer discussion forums tackle the question of whether it's appropriate for grown women to crush on an undead, underage hunk. The answer: a resounding yes. The responses—in this thread and many others like it—are a virtual "me, too" echo chamber. Women with kids and even grandkids confess, sometimes abashedly, their love for the saga. Many say their daughters turned them onto the books. One 65-year-old says she was forced to buy her own copies of the books after her granddaughter demanded them back. Others admit to giving their husbands the brushoff in favor of time with the fictional lovers.

So where's this blood lust coming from? Why have adult women been reduced to pilfering their kids' bookshelves and typing gushy online reviews that read like fan letters to Meyer's paper-and-pixel hunk? Some of the appeal comes from the escapism inherent in the series' romantic story line. Recessions are boom times for the romance genre; bodice-ripper standard-bearer Harlequin saw revenues rise by 2.2 percent in 2008.

Vampires are also enjoying a kind of pop culture vogue; in addition to Twilight, there's the Sookie Stackhouse series of graphic novels, written by Charlaine Harris, which have been turned into the HBO series True Blood. According to Simba Information's "Vampire Industrial Average," the number of best-seller titles featuring vampires has shot up from just seven in 2006 to 27 last year. While different editions of the same book appearing more than once can skew this average a bit, that's still a lot of blood-sucking.