Promising Young Woman by Caroline O'Donoghue

Jane is twenty-six, freshly dumped, and on the bottom rung of a lacklustre marketing career. She is also moonlighting as a worldly agony aunt. When an office party goes too far, she drunkenly tries out another role: the Other Woman.

As Jane's affair with her much older, married boss takes off, she disregards the advice her alter ego would give and dissolves into being someone else's dirty little secret. But she's not the only one at her company to have taken the wrong path. As she finds her own health and sanity disintegrating, can she discover the truth before another promising young woman is taken under his wing?