The poem that Harriet Vane reads in prison is 'To Althea, from Prison' by Richard Lovelace (1617-1657).
Harriet Vane: You're such a romantic, Peter. A sort of latter-day knight errant, searching for opportunities to perform deeds of valour, rescue damsels in distress ...
Lord Peter: What's wrong with that?
Harriet Vane: I've had a lover, Peter. I've been accused of murdering him and I'm not going to rush into another affair.
Lord Peter: Mariage.
Harriet Vane: Marriage. Affair. What's the difference? It's supposed to be love, isn't it?