This is a supplement for Ron Edwards' Sorceror.
It's a blood-soaked, post-apocalyptic fantasy setting in which the demons can be anything that has died or been forgotten. The bulk of the supplement is a dictionary of words that are important in the world, with a short rules section at the end. The rules cover what humanity is in the setting, some special rules for certain situations (e.g. falling in love, vendettas) which seem like they would add a little extra structure to the character interactions in the game, a bit more zing too.
Another element that I like is that if a player spends experience points or summons a demon, they must write up a new definition about it and add this to the dictionary. The supplement is really just a framework to build your game from.
The book is really evocative, with strange fonts, scrawled marginalia and strange doodles on the edges of the pages.
One of the problems I have with Sorceror is that it leaves pretty much all the setting creation to you. These days, I don't really have the time to prepare a setting from scratch, so I've only played it once, for a short run game. After reading Dictionary of Mu I am once again inspired - this seems to give me enough to build off, without being very restrictive.