In the beginning, there was Swanie.

H.N. Swanson, discerning a window of opportunity in Hollywood with the advent of talkies, opened one of the first Hollywood book offices in 1934 to co-agent books — by F. Scott Fitzgerald, Raymond Chandler and William Faulkner — for Gotham publishing agents.

Two generations later, book agents run a highly specialized industry within show business. As the spokes connecting Hollywood to the New York book market, their business is a vital part of the showbiz ecology. But it remains a uniquely tight-knit and migratory business.

It’s rarely seemed as tight-knit and migratory as last week, when ICM announced it had hired a former ICM agent, Tricia Davey, to work in its new London book office, and lit boutique Sanford-Gross announced it was splitting up; partner Geoffrey Sanford was teaming up with Sylvie Rabineau, who once worked as an agent at Sanford-Gross.

In fact, the career trajectories of most Hollywood book agents are deeply intertwined.

A remarkably high proportion of Hollywood’s book agents were mentored, directly or indirectly, by Swanie, who died in 1991.

Michael Siegel, Steve Fisher and Joel Gotler were Swanie apprentices — Gotler was hired after responding to a blind classified ad in Variety in 1976.

Siegel left to start his own agency, Michael Siegel & Associates, in 1992 then joined CAA and later Brillstein Grey, where he created a book division and hired Kassie Evashevski to help him run it. Siegel went out on his own again in 2000, and Evashevski stayed on.

Gotler is the connective tissue of a whole tribe of book agents. Endeavor’s Brian Lipson and BKWU’s Judi Farkas, among others, are former colleagues.

After going solo from Swanson, Gotler became a partner at Metropolitan Talent Agency, then started the Renaissance agency with Irv Swartz and Alan Nevins in 1993.

Renaissance bought Swanson’s estate in 1994, and the estate of Nevins’ mentor, Irving “Swifty” Lazar, one year later. Now Gotler has gone off on his own, and Nevins and Schwartz have stayed on at the Firm.

“All roads lead to Swanie,” Siegel said. “I think Swanie set up an imperfect but ultimately workable system that continues to this day. The rhythms got fixed in the ’30s.”

Not all Hollywood book agencies operate, as Swanson did, as co-agents for Gotham publishing agents. The West coast offices of William Morris and ICM, for instance, receive a continuos stream of material from their New York lit offices.

But CAA, which only recently opened a New York office, models its robust book division largely on the co-agenting system that Swanson employed. And virtually alone among Hollywood book agencies, it remains a citadel of homegrown talent.

Department head Robert Bookman, a vet of IFA and ICM and a former studio exec, was hired by Michael Ovitz to start the agency’s book office. His fellow agents — Matthew Snyder, Sally Wilcox, Shari Smiley, Laurie Horowitz and Brian Siberell — all began their book agenting careers at CAA. Smiley was recruited after working as a CAA receptionist.

By contrast, ICM’s chief Hollywood book agent, Ron Bernstein, has hopscotched across the industry. He came from Gersh, where he trained Angela Cheng Caplan, now a book agent at Writers & Artists. Before that, Bernstein worked at Lantz-Donadio and the Clients Agency. He was creative affairs director at Paramount under Robert Evans and he produced several telepics with Robert Stigwood in the 1970s.

Bernstein’s latest career move was part of a bizarre game of musical chairs among book agents. He joined ICM after Alicia Gordon ankled for WMA. Gordon followed Amy Schiffman, who was a book agent at WMA before replacing Bernstein at Gersh.

Why is there so much job-hopping among Hollywood’s book agents?

“With all due respect to everybody else, I think we’re all interchangeable,” said Lynn Pleshette, who runs her own lit boutique (her two former partners, Richard Green and Howard Sanders are now the chief book agents at UTA).

“You can be a brilliant agent,” Pleshette said, “but you’re only as good as the material you represent.”