United States publishers have long coveted the European market, and with Britain scheduled to formally part ways with the union by March 2019, some are gearing up for an invasion. A warning flare of sorts was fired last fall at the Frankfurt Book Festival, when Simon & Schuster’s chief executive, Carolyn Reidy, told an audience that, after Brexit, “the argument the British have used to grab Europe as an exclusive market will then be over.”

That’s about as close to a public declaration of hostilities as you can expect. The battle for Europe’s book market is already being fought quietly wherever author contracts are drafted. These contracts always include a list of territories where publishers want exclusive rights of distribution. More and more often, the list now drafted by American publishers includes the European continent.

Patrick Walsh, a literary agent based in London, has seen Europe included in early drafts of two deals with United States publishers. In both cases, the publishers argued that by the time the book was in stores, Britain and the European Union will have parted company. In both cases, Mr. Walsh pushed back, suggesting that it was too early to know how Brexit would play out, and the codicil was removed.

“It was a relatively friendly opening gambit,” he said. “But I’m absolutely certain we’re going to see more of this.”

Much of the talk here about the terms of Britain’s divorce from Europe has centered on banks, real estate and blue-collar industries like fishing and mining. But the creative sector is also bracing for change and fretting about life after Brexit. Few are as worried as the roughly 30,000 people employed by the British book business.

Part of it is a concern that Britain will cease to serve as one of the world’s great cultural hubs. London in particular has been a huge attraction for foreign artists in fields like contemporary dance and theater.