The Jagged Orbit, a novel by John Brunner (1969.) This is one of Brunner's 3 most "serious" books - which are often referred to as a "thematic trilogy." This one extrapolates racial conflict and arms sales to logical extremes. The other 2 are Hugo-winning Stand On Zanzibar (overpopulation) and The Sheep Look Up (pollution). This one easily makes my "top 100" list, and was a Nebula-nominee in 1969. It is the kind of book that I will cheerfully sell as many nice copies as I can find.

Since I read it years ago, I am relying on a review by P. Schuyler Miller from ANALOG September 1969 for a coherent plot summary:

"In this book he is exploring the possiblities of Black-and-White confrontation ... it's another beautiful job - intricate yet tightly constructed,thoroughly believable, with as many important characters as a Victorian classic. John Brunner just doesn't buy the artificial limitations of the '1 book, 1 character' concept. The United States of the next century has split up into Black and White enclaves - some separate cities, some encapsulated withing cities - which are mutually antagonistic and mutually dependent. Cosa Nostra has become thoroughly integrated and even more internationalized; it is the planet-strangling Gottschalk weapons combine... The Gottschalks sell weapons of every kind and by every possible means, to individuals - and they are merely supplying a society of corruption and violence that wants and needs their wares. In and out of this strange fabric we follow the threads of several independent yet interdependent people. Matthew Flamen is a spoolpigeon, last of his kind in New York, descendant of our TV commentators but with a vastly more powerful technology at his disposal. Computers check the "truth" of his news - but other computers simulate what he shows as happening. His wife is in a State mental hospital where other strange things are happening. There is one very strange black technician. There is a visit from a pythoness - a seer - whose visions are far from cryptic. There is Diablo, Flamen's black counterpart, hounded out of Blackbury and seeking a refuge in the White community he hates. There is Xavier Conroy, the book's philosopher-in-residence and perhaps spokesman, or devil's advocate, for the author - another good Victorian technique that he uses effectively. Glimpsing them, watching them, following their we see their world and watch the crisis building, even though we have no idea what is really happening. And then - Well, that's something for you to find out for yourself. Read Conroy's blast on pages 337 to 340 [of the paperback edition], look around you, and then buy the book and read the rest."