Dry World
This is a rules-light¸ post-apocalyptic and most of all godless setting idea.
3 races¸ 6 classes and an unforgiving world¸ rendered through irreverent pixles and sketched maps.
Choose who you want to be: a tough commie¸ a sneaky mutant or a nanotechnological psycho.
Face lovecraftian horrors¸ desert cannibals and ineluctable radiation in the Dry World.
Forget the old judeo-christian absent gods¸ and kneel to inscrutable angel-like aliens without prophets¸ hoping they let you live the entirety of your fixed lifespan.
You don't need this book. It has been developed by grown-up kids for grown-up kids. Most of it is a cue¸ left open for the Designer to develop for their personal use. The rules are minimal and the maps are sketched¸ but you'll have fun building upon it.