The Number of Love by Roseanna M White
Three years into the Great War, England's greatest asset is their intelligence network--field agents risking their lives to gather information, and codebreakers able to crack every German telegram. Margot De Wilde thrives in the environment of the secretive Room 40, where she spends her days deciphering intercepted messages. But when her world is turned upside down by an unexpected loss, for the first time in her life numbers aren't enough.
Drake Elton returns wounded from the field, followed by an enemy who just won't give up. He's smitten quickly by the intelligent Margot, but how can he convince a girl who lives entirely in her mind that sometimes life's answers lie in the heart?
Amid biological warfare, encrypted letters, and a German spy who wants to destroy not just them but others they love, Margot and Drake will have to work together to save themselves from the very secrets that brought them together.
The Codebreakers Series, Book #1