Contending Forces by Pauline Hopkins

From Union Square & Co.:

Sappho Clark—beautiful, mysterious, Southern—arrives in Boston to earn her living as a stenographer. She lodges with the Smith family and immediately becomes a source of fascination to the them: Ma Smith is impressed by Sappho’s financial independence; Dora Smith admires Sappho’s quiet self-possession; and Will Smith, Dora’s brother, falls madly in love with Sappho. But as Sappho enters the Smiths’ community, it becomes clear that her beauty is a lure to bad actors, including someone who entertains dark suspicions about her past. . .
 
A murder mystery, the story of a friendship, and a romance set in Boston’s thriving, politically active middle-class Black community, Contending Forces is an unjustly forgotten American classic.