It is an alternate history revolving around the experiences of identical twin brothers during the Second World War, during which one becomes a pilot for the RAF, and the other, a conscientious objector, becomes an ambulance driver for the Red Cross.

British magazine The Bookseller reported in May 2003, that The Separation was "to be republished by Orion after the author bought back the rights from Simon & Schuster."

The article added that, in November 2002, Orion's managing director, Malcolm Edwards approached Priest and proposed that Gollancz republish the title in hardback.

"[2] Elizabeth Hand described the book as "exquisite ... an exceptionally frightening novel whose nightmare power derives from its chilling, almost clinical evocation of an historical reality with which we are all familiar, the London Blitz... a cliffhanger narrative of dual identities, betrayals, and shifting realities, as two versions of the twins' histories—and England's, and the world's—are woven together, like strands of DNA, to form a terrifying narrative.

Many alternative history novels are bloodless extrapolations from mountains of data, but this one quietly builds characters you care about—then leaves their dilemmas unresolved as they try to believe that what they have done is 'right'.

"[4] Pauline Morgan, reviewing the novel for SF Crowsnest.com, said, "This is a glorious book to read—not for nothing was Christopher Priest included in the line-up of Britain's best young novelists some years ago.