Joelle Delbourgo, editor-in-chief and associate publisher of HarperCollins's adult trade division, has resigned from the company, effective at the end of May. No reason was given for her departure and no one was named to replace her at press time last week. Delbourgo joined HC in early 1996, after serving as editor-in-chief of Ballantine. While at Harper, she helped launch the literary imprint HarperFlamingo. She can be reached at (973) 746-1078.