A documentary novel is a novel - fiction. I'm no expert on nuclear war, but imo once the major powers start throwing nukes, they'll throw a lot of nukes. The Trident SLBM is the MIRV weapon AFLM mentioned on pg. 2 of this thread; it has 12 independently targeted nuclear warheads. There are 24 Trident missiles on an Ohio class submarine giving 288 thermonuclear warheads per submarine. There are 18 active Ohio class submarines, I have no idea how many are operational at sea, but let's say 1/3 of them as a point of reference. Six submarines times 288 warheads is 1728 warheads.

That's a lot of destruction and potentially a lot of radioactive fallout. Then there are the land-based missiles which in a nuclear exchange will be used first (use 'em or lose 'em). Then there are the Russian, Chinese, Pakistani, Indian and Israeli (assuming they exist) nuclear arsenals -- there are a lot of nukes out there. 2 billion dead in the initial days of WWIII going nuclear is reasonable, the death toll will climb from there and it will not discriminate.