Code Name Sapphire by Pam Jenoff; 2023. Softcover

1942. Hannah Martel has narrowly escaped Nazi Germany after her fiancé was killed, taking refuge with her cousin, Lily, in Brussels. Desperate to flee Europe, Hannah joins the Sapphire Line: a secret resistance network led by a mysterious woman named Micheline and her enigmatic brother Mateo. But when Lily's family are arrested and slated for deportation to Auschwitz, Hannah finds herself torn between her loyalties and her own determination to escape. How much is Hannah willing to sacrifice to save the people she loves?