Dietz is not a great writer. But then, I don't think he wants to be a great writer. What he does do (reasonably well) is put together tight thrillers that focus on one social ill, wrapped up into a SF setting. And he usually can spin a decent yarn. Probably his best novel is Where The Ships Die, which mixes an espionage thriller with an image of the great Indian ship breakers working on starships. DeathDay is not that novel. If anything, it's Dietz's worst piece to date.
It's 2020 and in a scene straight from Independence Day, the Earth has been invaded. Billions are dead, and the scattered remnants of humanity have been enslaved, to build enormous temples by hand. It doesn't look good, and Will Smith doesn't seem to be on the scene. Set around Dietz's home in the Pacific Northwest of the USA, DeathDay chronicles the first struggles of a resistance movement, and humanity's discovery of the real reason behind the invasion. We see the broken world through the eyes of slaves and rebels - including the doctor in a slave camp, a history teacher who finds himslef at the head of a resistance movement, and the head of the security team protecting a quisling leader.
Dietz struggles to wrap a message about racism into his story, with a look at colour-based slavery (with a reversal of history as an ironic twist to drive a point home) on the invaders' part, and as neo-Nazi survivalists try to control the resistance - one thing's for sure here, Dietz is not subtle. It's a fair attempt, but it's one that's been done far better by other writers. Dietz has ambition, but his reach far exceeds his grasp. His characters are 2D cutouts, who are pushed through the plot by editorial fiat.
There is a sequel, which surprisingly seems to be in hardcover...
A reasonable Clancyesque pulp, with some interesting ideas. But I'd suggest reading William Barton's superb When Heaven Fell instead for a much better treatment of an Earth under the yoke of unfeeling alien masters.