The latest black eye: Dinaw Mengestu, a novelist who became PEN America’s president last December, resigned on July 9 and then, in an essay for Literary Hub, attacked “the organization’s hypocrisy,” “dishonesty” and “bigotry.” As in past PEN America controversies, the flash point was disagreement over Israel and the Palestinians. The underlying causes are the literary world’s expansive sense of its social role and confusion about what counts as a threat to free speech — and what doesn’t. . . .