Stupidly I don't know Furst-style resistance fiction that well -- maybe Modiano? Or Laurent Binet's 'HHhH'? Both of which are a bit more ambitious than Furst. Or Hans Fallada's 'Every Man Dies Alone'?

My standby broader literary spy recommendations are David Quammen's extremely underlooked 'The Soul of Viktor Tronko' which is a sort of American 'Tinker Tailor,' and Joe Weisberg's 'An Ordinary Spy.' There's also Javier Marias, who's great but is maybe too arty for brothers-in-law...