The Sealed Letter by Emma Donoghue

Based on a real-life scandal that gripped England in 1864. From the bestselling author of Room, Emma Donoghue's The Sealed Letter is a delicious tale of secrets, betrayal, and forbidden love.
Helen Codrington is unhappily married. Emily 'Fido' Faithfull hasn't seen her once-dear friend for years. Suddenly, after bumping into Helen on the streets of Victorian London, Fido finds herself reluctantly helping Helen to have an affair with a young army officer.
The women's friendship quickly unravels amid courtroom accusations of adultery, counter-accusations of cruelty and attempted rape, and the appearance of a mysterious 'sealed letter' that could destroy more than one life . . .
'The Sealed Letter is a page-turner with a jaw-dropping ending' - Stylist