If you've read the "50 Shades of Grey " erotic trilogy, then you know that Audis make an appearance throughout the books nearly as much as sex toys do.

Given that few other brands appear in the books, Audi's feat of securing a starring role for not just one of its models, but for several different cars (in addition to the A3, others that make appearances are the R8 Spyder, and the Q7 sport-utility vehicle) is pretty astounding. Even better, it was all for free.

Audi told Ad Age last week that not only did it not pay a dime to be part of the book series, they had no involvement or knowledge of the tie-in beforehand and were stunned when they found out about the write-in of the cars once "50 Shades" began to pick up buzz.

The books are estimated to have sold over 30 million e-book and print copies in English alone, with book rights sold in several countries. And the series -- which has become a sensation particularly among middle-aged women, earning the nickname "Mommy Porn" -- is also believed to be one of the fastest-selling paperback tomes of all time, though it still pales to J.K. Rowling's blockbuster "Harry Potter" series, which has sold upwards of 400 million copies total.

What's more unusual is that the books' author, E.L. James, isn't even an Audi owner or huge fan herself. Her director of publicity at Random House Group, Russell Perreault, told Ad Age that "She does not drive an Audi. She drives a Mini and her husband drives a Volkswagon Touareg."

So how'd it happen? In a rare instance, it was a totally organic branded-placement that came about merely because Audi stayed true to its brand positioning as the high-end, luxury car of choice for users who want both design and performance.

It's a standard that the carmaker has focused on achieving over time, and something that 's underscored by third-party praise, with Audi taking home award after award. Earlier this year, Audi received Kelley Blue Book's highest honor, topping all other luxury brands, when the 2012 Audi A5 was selected as best luxury car, and 2012 Audi Q7 chosen as best luxury sport utility. More recently, it was named the Highest Premium Brand for the Autobytel and AutoPacific 2012 Ideal Vehicle Awards, and swept the category for "aspirational luxury car."

Rob Donnell, the founder-president of Los Angeles-based Brand Arc, a branded entertainment firm that has a lot of experience doing branded deals for cars, such as Toyota, believes this is a rare instance of a branded tie-in that is driven by product attributes and how they weave into a story's plot.

"It was probably purely character-driven," said Mr. Donnell. "Cars always define character quite precisely, and that 's usually one of the ways [brands] can get in early" on books, TV shows or movies. When deals can sprout up organically and later lead to a paid arrangement between a brand and an entertainment property can be ideal. But it's not so common these days, he said. "You see this less more so these days than five years ago because everyone knows that money is to be had [from brands]."

Says the female protaganist of "50 Shades," the 22 -year-old virginal Anastasia Steele, after being gifted an Audi A3 coupe by her rich and domineering boyfriend, Christian Grey :

"I can drive the Audi in high heels! At 12:55 p.m. precisely, I pull into the garage at Escala and park in bay five. How many bays does he own? The Audi SUV and R8 are there, along with two smaller Audi SUVs...hmm. I check my seldom-worn mascara in the light-up vanity mirror on my visor. Didn't have one of these in the Beetle."

For Audi, the "50 Shades of Grey " unpaid placement comes as its sales are on the upswing, increasing in the double-digits. It set an all-time U.S. July sales record with 28% increase -- which represented 11,707 vehicles -- which also was the brand's 19th consecutive month of record sales.

The company says it doesn't have any data that proves that it's reaching more women or seeing an uptick as a result of "50 Shades," but they're convinced that the halo effect has been and will be pretty significant.

"As Audi continues to generate momentum in the U.S. market, we're seeing our products utilized more and more within popular culture," said Loren Angelo, general manager of Brand Communications. "Those organic results are always a sign that a brand is generating real cultural relevance. Obviously, the runaway success of this series has introduced Audi to consumers who might not have previously been thinking of the brand and that certainly has a positive impact for us."

It's pretty great visibility at a time when the carmaker -- for the first time in years -- doesn't even have a chief marketing officer. Earlier this year its CMO of six years, Scott Keogh, was promoted to president and the company has yet to name a new CMO.

It should be noted that during the course of the three books, a couple other cars get mentions too; the Saab 9-3 was purchased for Ana after her Audi A3 was damaged, and her best friend Katherine drives a Mercedes CLK. But the Audi gets most of the love, and that 's evident with how the Audi has crept into the pop culture and social media memes around "50 Shades of Grey ."

On fan sites for the books the Audis are mentioned, and Pinterest pages dedicated to "50 Shades" have pinned images of the cars.

The Edgewater Hotel in Seattle, one of the cities where much of the books take place, even created a promotion to bait consumers inspired by the books that includes waterfront accommodations, champagne and a demo drive of an Audi through a partnership with a local car dealership.

While that sort of attention's been great for the brand, the real potential lies in the possibility of Audi being written into the forthcoming film version of "50 Shades of Grey ." A couple years ago, Volvo milked its tie-in with the tween vampire saga "Twilight", creating a viral-video campaign by EuroRSCG and promotional giveaways.

And Audi, which in the past has been willing to pay big bucks to be a part of big entertainment events, such as the Super Bowl and "Iron Man 2," might be willing to pay up going forward.

Universal and Focus Features bought the rights to Ms. James' trilogy for $5 million, and the producers behind "The Social Network," Michael De Luca and Dana Brunetti, are also signed on. Ms. James has said it's too early for casting, but the most likely one to get a role at this point is Audi.

Brand Arc's Mr. Donnell predicts the movie will likely not veer from the brand that 's been written into the books.

We were pitching a movie and the book had the car written in as a different car and the filmmakers decided they wanted to stay true to the book," he recalled. "'50 Shades of Grey ' has been such a sensation -- I think my wife read all three books in a week -- that when you know you have such a dedicated audience who are going to look at the film with the scrutiny, it's more likely than not the filmmakers will say let's keep it the way it is ."