I'm assuming that these "Hell on Earth" books are part of the d20 version of Deadlands, correct? I've considered Deadlands before but I've never felt a desire to look at the setting in detail. Is it "Old West" meets "apocalyptic zombies?"
No, no. It's part of their original line and only has zombies as an incidental feature. Specifically, it's their Science Fiction Apocalypse setting. Essentially, the Deadlands: Weird West setting was advanced about 200 years until there was a massive nuclear war between the Union (and allies) and the Confederacy (and allies). Which is when the Four Horsemen of the Apocalypse decided to invade the real world.
It gets quite messy and apocalyptic for a while, but the short version is that the only inhabitable land on the planet is west of the Mississippi River, and there's just pockets of civilization left to fight the fear. Oddly enough, there's still a small sliver of hope for actually saving the world, if the party can overcome cyborgs, robots, demons, the living, the dead, and their own twisted natures to defeat key evils and spread real hope to the rest of humanity. Piece of cake.
It combines just about every science fiction trope there is, from kung-fu to psychers to cyborgs to armies of murderous robots (some might call them terminators), and blends them with the style of the Old West for a really iconic and unique setting. There's magic, undead, demons, devils, a few living saints (miracles and all), and road gangs straight out of Mad Max. You've got vaults full of special forces soldeirs put into cryo-stasis in case the world exploded, the remnants of Chinese army units keeping an American town safe and prosperous, and street cops right out of Judge Dredd policing Las Vegas. Heck, they've even got aliens, from the one serious attempt at a human colony, but only one or two aliens maybe came back to Earth (thank all that's holy).
Hover tanks, powered armor, Sioux warriors riding horses, and soul-eating rains. Hell on Earth lives up to its name, and is a fantastic post-apocalyptic setting. The mechanics are good for the parent rules, but can be completely ignored when reading the books.