Poirot hasn’t yet said who wrote this letter to Evelyn, but the letter itself reveals that Evelyn Howard was an accomplice to the murderer. It was, the letter clarifies, Evelyn’s idea to use bromide powder to crystallize the strychnine in Emily’s tonic, thus ensuring that she would take a lethal dose at the bottom of the bottle. Of course, it’s not yet clear
why Evelyn would want to kill Emily, especially since she’s (supposedly) her best friend, but the fact that she’s guilty of helping kill the old woman
does make sense of her behavior earlier in the novel—Poirot, for his part, noted early on that her vehemence toward Alfred Inglethorp seemed suspicious.