Strong Poison is a 1931 novel by Dorothy L. Sayers, her fifth featuring Lord Peter Wimsey.
It is in Strong Poison that Lord Peter first meets Harriet Vane, an author of detective fiction. The immediate problem is that she is on trial for her life, charged with murdering her former lover — if Lord Peter does not prove she is innocent, he will lose her before he even persuades her to accept his proposal of marriage. But all the clues point to Harriet as the one who gave Philip Boyes the arsenic that killed him.
Strong Poison was adapted for television in 1987 as part of a series starring Edward Petherbridge as Lord Peter and Harriet Walter as Harriet Vane.