Kommandant's Girl by Pam Jenoff

Emma Bau has been married only three weeks when Nazi tanks invade her native Poland. Within days her husband Jacob, a member of the Resistance, is forced to flee, leaving her imprisoned within the city's decrepit, mouldering Jewish ghetto. But then, in the dead of night, the Resistance smuggles her out, and she is given a new identity as Anna Lipowski, a Gentile.

Emma's already precarious situation is complicated by her introduction to Kommandant Richwalder, a high–ranking Nazi official who hires her to work as his assistant. Urged by the Resistance to use her position to access details of the Nazi occupation, Emma must compromise her safetyÐand her marriage vowsÐin order to help Jacob's cause.

As the atrocities of war intensify, so does Emma's relationship with the Kommandant, building to a climax that risks not only her double life, but also the lives of those she loves.