Dan Brown is a mega-star with four bestsellers and vast pop-cultural influence. Sheryl Jane Stafford is an unknown thriller writer self-published with Writer's Showcase Press. She has, it seems safe to say, less cultural influence.

But the two are a natural fit, at least according to Amazon.com, where Stafford's A Deadly Exchangeis the only title to be packaged with Brown's Angels and Demons—and the only book for which the purchase of Angels & Demons earns customers a special discount. The promotion is not the result of any editorial decision on Amazon's part; instead, the book landed in that spot because Stafford deciphered a little mystery of her own: she figured out she could pay for it.

In a little-known but eyebrow-raising program, Amazon is pairing well-known authors with nearly any publisher or author willing to pay the fee. It's a program that evokes strong reactions: while the site's willingness to let unknowns mix with celebrities is hailed by small publishers, others say it raises significant questions about Amazon's responsibility to protect an author's brand or to disclose its own financial arrangements.

The program, known as BXGY to publishers (for Buy X Get Y) and appearing to customers under the header "Best Value," has existed in some form for several years. But anecdotal evidence suggests a recent increase in small-publisher participation. Under the program, houses or authors interested in pairing with a bestseller are sent a list of eligible bestsellers, which, according to participants, includes nearly every author who isn't taken. They can then pay $750 per month to be linked with one of Amazon's bestselling authors, sometimes paring for multiple months and rotating among several bestsellers. The program is distinct from the similar-looking "Better Together," where the choices are made, without sponsorship, by Amazon—though no such distinction is made on the page.

BXGY can function like traditional publisher co-op; Nan Talese has used the opportunity to bundle Ian McEwan's Atonemen twith his new novel, Saturday. But often an unusual book will lock up the spot. Michael Crichton's current partner for State of Fear is Fatal Memories by Vladimir Lange, a heretofore unknown author at the heretofore unknown Red Square Press. Dan Brown's Deception Point is sold with a self-published mystery called The Scylla Hexagram. Jared Diamond's scholarly but accessible Collapse draws History:Fiction or Science, which is described in part by its publisher, an outfit called Mithec Press, as "Eminent mathematician proves that: Jesus Christ was born in 1053 and crucified in 1086."

The chance to be associated with some of the world's best-known authors has even respected small houses excited. "It's a terrific cross-merchandising opportunity," said Esther Margolis, publisher of New York's Newmarket Press. And the tool can indeed have a strong sales effect. Texas's tiny Synergy Press saw its Ingram sales double in the month it paid to have its thriller Master of the Game next to James Patterson's Honeymoon. "I understand that a large house might be concerned," said Amanda Willis, Synergy's publishing coordinator. "But from our perspective, it's really something that levels the playing field."

Still, the specificity and intimacy of an online pairing goes beyond traditional co-op to create the wrong impression, according to some. "It just seems deceptive to take a serious book and put it with some of these books," said the agent of one affected author. Without any veto power from a publisher, added one marketing executive, the program "puts the author in a terrible position."

Asked about the extent of Amazon's responsibility for quality control or disclosure, spokesperson Kristin Mariani said customers "know we act like a bookstore and are aware that some placements are paid for." She also said that, to her knowledge, no publisher had ever complained about a pairing, adding that the company is happy with the program and expects it to continue.

Observers say the program reflects several tensions particular to Amazon, including the site's hybrid status as both a store and an editorial entity, and the vagaries of co-op when each title page is essentially a separate window display. They also question the program's business logic. "I'm not really sure what it does for Amazon. Are they making more on co-op than they would if they placed a book that might be more appropriate?" said the agent. "It's like Google. The way these things work is you learn to trust the site. When you have something like this, it just seems weird."