Pete Townshend had Who Came First. Beck had Mutations. Jeff Tweedy had Loose Fur. For rock stars, side projects are a tradition as honorable as the farewell tour and detox. But it turns out they're not just for musicians anymore. Big authors at big houses are now taking little detours of their own, often in the middle of a productive career.

A few months ago, Nick Hornby stepped away from Riverhead to do a collection with Believer Books. Caleb Carr has been writing a mystery for Carroll & Graf even as he's supposed to be working on his next Alienist book for Random House. Doubleday franchise Jonathan Lethem has gone contemplative with a McSweeney's novella. And last week, Stephen King announced that he's taking a break from Scribner, which over the last seven years has published him on subjects ranging from writing to Red Sox, to do a pulp thriller with a paperback startup called Hard Case Crime.

The book world has long had its wandering authors; Nora Roberts has made a living on spreading the love. But side projects are different. Authors who have them don't sign longstanding contracts with several big houses—they make one-time exceptions with small ones. The modern archetype: Walter Mosley, who in 1996 heard an activist urge black authors to publish with minority houses, then told Norton he was taking time off for tiny Black Classic Press.

So should the jilted publisher be offended? More important, should it be worried?

While most houses don't have a choice in the matter, they do have reason to be uneasy. These days, a publisher spends serious advance and marketing money not just for an author's work but for his or her brand—a brand that a side project can put a new (and perhaps unwelcome) stamp upon. After King's announcement, Scribner's Susan Moldow was quick to point out that her house would still do the book in audio and electronic formats, and might eventually include it in a collection. "It's a one-time deal," she declared. In other words, it's just a side project.

And the concerns continue right through publication. As publishers frequently remind us, it's the author, not the house, that readers notice. That's why, after some early anxiety, Norton made a marketing and publicity push for Mosley's outing despite the lack of an immediate financial stake. "It's a big concern for a publisher in a world where an author is perceived to be only as good as his last publication," said Mosley's then-editor Gerry Howard. "It was certainly in Norton's interest to keep the numbers as high as possible."

The newly blessed house has its own reasons to be careful. Both Charles Ardai of Hard Case and Will Balliet of Carroll & Graf said they were "thrilled." Veterans, however, warn that eye can be bigger than stomach. Black Classic's Paul Coates was said to be talking about the dangers years after his Mosley experiment. "A publisher might have to cover a printing bill that's larger than its production bill for an entire decade," Howard noted.

The Restless Ones

But the issue of publisher reaction begs the larger question: in this era of unprecedented reward and resources for celebrity authors, what makes one want to jump in the first place?

For some, of course, it's the chance to step out of the limelight and into a more free-spirited space, the literary equivalent of putting on a Halloween costume. "There's a lot less weight of expectation," said Carr agent Suzanne Gluck, referring to her client's C&G book, a Sherlock Holmes mystery in the voice of Dr. Watson. "[Random House] has a number that they have to meet based on what he's done before." But here, "We're opening an indie film, and we don't open on 800 screens. We open in select theaters."

In King's case, sources say it's his way of giving back. He offers shout-outs to younger writers and collaborates with lesser-knowns. Now he's handing a bestseller to a startup.

And there's a more general reason: for the last few years, publishers have encouraged authors to become multimedia dynamos. Gone are the days when an author surfaces with a manuscript every few years and disappears again soon afterward. Today, even fiction authors are practically incorporated entities. Nick Hornby has film deals, newspapers columns, speaking engagements and novels. Why wouldn't he try another publisher once in a while?

But the precise trajectory of these projects also points to a more specific, and perhaps radical, idea.

Nearly all of the authors with side projects have gone from muscular but exceedingly general big houses to tightly focused special-interest ones. C&G is a Holmes hotbed in a way Random could never be; Hard Case does pulp more aggressively than anything S&S could ever pull off; the sly humor suggested by every book with a Believer colophon couldn't be matched by an entire Penguin division.

Has Big Publishing become so conglomerated that it has made franchise authors—the very authors these houses were presumably conglomerated to serve—want to turn away? And not just away, but toward the specialty publishers these big houses were supposed to make irrelevant for celebrities?

Large publishers, executives remind, will always hold a trump card. "The big authors all want the big payday," said one. "And they're just not going to get that from the smaller guys." Maybe so. But with all that stardom flying around, sometimes they also just want to jam a little.