Joshua Wolf Shenk, author of the books “Lincoln’s Melancholy” and “Powers of Two,” has resigned from his positions as editor in chief of the Believer magazine and artistic and executive director of the Black Mountain Institute. On March 24, staff of the institute, a literary arts center within the University of Nevada, Las Vegas and the publisher of the Believer magazine, were notified via email that Shenk, 50, had resigned and that John P. Tuman, associate dean for faculty at the university, would step in as its acting executive director. A search for a new executive director will begin “as soon as is feasible,” wrote Jennifer Keene, dean of the College of Liberal Arts.