Sixteen years after readers were introduced to the magical world of Jonathan Strange and Mr Norrell, Susanna Clarke is to publish her second novel.

Out in September next year, Clarke’s Piranesi will follow the story of its eponymous hero, who lives in the House, a building with “hundreds if not thousands of rooms and corridors, imprisoning an ocean. A watery labyrinth.” Occasionally, he sees his friend, The Other, who is doing scientific research into “A Great and Secret Knowledge”. Piranesi records his findings in his journal, but then messages begin to appear, and “a terrible truth unravels as evidence emerges of another person and perhaps even another world outside the House’s walls,” said Bloomsbury, which announced on Monday that it had acquired the novel in a two-book deal.

Clarke’s debut, Jonathan Strange and Mr Norrell, took her more than 10 years to write. Running to more than 1,000 pages, it is set in an alternative version of 19th-century England, where magic has almost – but not quite – faded into the past. Published in 2004, it took the world by storm, selling more than 4m copies, winning prizes including the Hugo and World Fantasy awards, and was adapted by the BBC in 2015.

Since then, the British author has published only a collection of short stories set in the same universe, The Ladies of Grace Adieu, telling interviewers that persistent ill-health had slowed her down.

Clarke’s agent, Jonny Geller, said that there are “a few moments in an agent’s life when something so unexpected and so wonderful pops up in your inbox, you can’t quite believe it”.

“Susanna hinted she may be writing again after such a long hiatus, but I never really believed a fully imagined world, a perfectly constructed novel, would just be sitting there,” said Geller.

Clarke’s UK editor, Alexandra Pringle, said that she had thought that Jonathan Strange, which “appeared from the ether like an apparition … couldn’t be equalled”. “But when I followed Piranesi into his watery Halls, I discovered Susanna’s wit, strangeness and sorrow made new and beautiful in ways I could not have imagined,” she said. “It will be an honour to open the doors to the House, and show its beauty to the world.”