The Last Hope by Susan Elia MacNeal

In this final book of the Maggie Hope series, it's 1944 and Maggie is asked to go undercover by fellow spy Kim Philby to discover how far Germany has progressed in building an atomic bomb. Maggie has to go to Madrid and get close to Nobel prize winning German physicist Werner Heisenberg. If Heisenberg is building the weapon, Maggie must then assassinate him. Travelling to Spain, Maggie encounters both friends and foes, including fashion designer Coco Chanel, and it's hard to know whom to trust. Will Maggie get the happily ever after she deserves?