What is the cost of keeping a secret from someone you love? In Berta Isla, a novel by the celebrated Spanish author Javier Marías, a couple’s minor disloyalties – which they justify as mere ‘parentheses’ or emergency measures in their relationship – breed further betrayals until they find their lives ‘swathed in mist and mystery’. When does an extended parenthesis overwhelm the sentence in which it occurs? How long can you lead a parallel life before it supplants your real one? At what point does a state of emergency become, simply, the law? These questions all bear upon the action of this guileful and beguiling tale. Like its characters, Berta Isla is a shape-shifter: part spy thriller, part murder mystery, part cerebral caper with echoes of Melville, T S Eliot and Henry V. It is also a trans-European historical novel, set between Oxford and Madrid in the 1970s and 1980s, with a hinterland that includes the campaigns of ETA and the IRA, the fall of Franco, the rise of Thatcher and the Falklands War.