The Genocides is an underrated book. It was a paperback original, his first novel before he had acquired much reputation (it was published in 1965), and his next two novels were undistinguished (I think Camp Concentration is the fourth, not counting work-for-hire stuff). The Genocides was also vilified by a couple of reviewers, including Algis Budrys, who dismissed it as a J.G. Ballard imitation (and we were all supposed to know how we felt about that back then; the war between the traditionalists and the radicals was already beginning to heat up). But it's a compact and neat story of collapse, and the character delineation is superb. Disch has said that to his mind, the appeal of the disaster novel was the implacability, and his disaster wasn't going to be diluted by any kind of rescue or redemption. Even so, it isn't bleak, exactly; the narrative follows the characters and their emotions and tribulations like a camera, but there is something about it that feels disengaged, so the story isn't sad or tragic or even bracing; it's just there. In that sense it is like Ballard, except Ballard's disaster novels tended to suck all human emotion out of everything except one character's interaction with his disaster.
Another thing that keeps it from being totally bleak is the rich vein of black humor running through it. That said, it's still pretty bleak. But a great book regardless.