Ave Molech is a fantasy steampunk setting created under the Open Games Licence. It's actually been loose since 1997 and this is the Second Edition, although regretfully this is the first time I've seen it. The Introduction helps, as it is a reminisce by the author about how he got into role-playing and how the Ave Molech setting came about.
Chapter 1: The World of Ave Molech sets out its stall: a post-cataclysmic mediaeval fantasy world with steampunk inventions and a Wild West attitude; and recommends that not only the core Dungeons & Dragons 3e books but also D20 Modern be used. Due to the catacysmic events of the past, much of the population has been living underground or floating high above, and only the past couple of hundred years has seen more than the most adventurous venture out on the surface. Now both magic and technology are being harnessed to make the surface a fit place on which to live again. Much knowledge, especially the detailed history of what took place, has been lost; but some remains to be discovered. A brief history describes what little is known of how the cataclysm came to occur, how it opened the world to external invasion, and how matters developed thereafter until the present day. Then comes a couple of 'eyewitness' reports and a timeline of major events.
Next, the current state of affairs is described - basically, fragmented outposts of various sizes each doing their best to survive against the remnants of alien invasions and hostile neighbours. However, different groups bring a range of talents, skills and knowledge to the world community as a whole - there is even a primitive Internet to enable different settlements to keep in touch with one another! Magic exists alongside technology, being of the familiar arcane and divine persuasions as well as 'ancient magic' from which all else stemmed. Most divine magic comes from a belief in oneself rather than in an external deity, as veneration of gods seems to have vanished during the cataclysm and its aftermath... and the gods themselves appear to have gone to sleep. Various other topics, ranging from psionics and planar travel to fashion, economics to body-art and piercing, transportation and more are also discussed to present an overall picture of Ave Molech as it is today.
Chapter 2: The Races and Lands of Ave Molech gets into a bit more detail than the overviews already presented. On races, any fantasy race can exist here, but the most common are humans, goblins, tiefling, halflings, half-elves, half-orcs, half-giants and the sec'toda. Full game details for these races as they appear in Ave Molech are provided. Few pure-blood elves remain. Sec'toda appear human, adapted to subterranean living, but they are not - instead they are the outwardly-visible partner of a symbiotic relationship with a 'bloodmate.' Being a new race, they are described in greater detail than the others.
Next comes a gazetteer section, describing the cities, swamps, coastal areas and other parts of the world. The level of information is sufficient for at least a visit to each location mentioned. The maps presented are somewhat rudimentary but provide a good overview of the world with plenty of scope for individual DMs to develop areas of interest within that context.
Chapter 3: Organisations and Important People puts some meat into the setting, providing details of various groups and individuals with which characters might interact - or whose wider machinations may well impinge on their lives. It's a setting in which both exploration and intrigue can flourish, depending on your group's tastes.
Next comes Chapter 4: Game Mechanics begins with a look at character creation. It's suggested that the D20 Modern methodology be used for such as ability scores and core classes, with amendments as appropriate to fit the setting. However, skills and feats from both D20 Modern and the D&D Player's Handbook, modified as necessary, can be taken. The DM should judge, based on the type and focus of campaign, just what will be available.
Just to give you a start, though, Chapter 5: Advanced and Prestige Classes goes into much more detail to enable progression to be planned and characters developed. Each in turn is presented with the necessary information to enable them to be used. They include members of several of the organisations mentioned earlier as well as more general classes that do not require you to sign up to a specific group. One intriguing prestige class is the Dragonne, who have mastered the use of technologically-advanced battle armour as well as more mundane combat skills. Sneakier folks might prefer to become Shadow Dancers, while a Fearasitic Mage is as scary to his enemies as he is inspiring to his friends. In a land without religion, the dead (or more correctly, the undead) might become a problem, hence the Ecclesiarch class, specialist in controlling them; while for more mundane problems, you might want to send for a Bounty Hunter.
Characters sorted, Chapter 6: Equipment, Vehicles and Other Ware looks at the kit your characters might acquire. Note that the D20 Modern wealth system has been used, so those desiring more realistic economics will have to devise their own price lists. There's a range of delightfully exotic personal equipment which is described in enough detail for you to know what it does without necessarily needing to know how it does it: perfect for the steampunk-style tech in this setting. Mechanical vehicles, ships and airships, wonderous items... even piercings that deliver a magical effect... enough is here to get you started. Oh, and there are some exotic riding animals if you prefer creatures to mechanical wonders.
Worthy of a chapter of its own, next we have Chapter 7: Clockwork. Developed to a level to rival real-world electronics, clockwork can be used to power and control devices of all natures, limited only by the inventor's imagination. One major use is in Clockwork Companions, in other words, robots - but not advanced enough to be living or self-aware. The chapter concentrates on the manufacture, programming, operation and repair of these constructs.
Next is Chapter 8: Monsters, detailing the wildlife - or at least three of the more dangerous species - inhabiting Ave Molech. You really don't want to encounter any of them! This includes more detail on the main alien invader, the Hybrude - including background that is not known to natives of Ave Molech. There are also notes on quite a few other creatures that you might encounter, although you can of course use many of the creatures in the Monster Manual series of books as well... although they may have been warped on their way there. Chapter 9: Creature Templates provides some of the tools, with a selection of templates you can apply to the monster of your choice.
Chapter 10: GM Stuff lifts the curtain on some of the underlying concepts, and shares things like the house rules and mechanical approaches which the author has found best reflect the flavour of this setting. In terms of general gaming, there is a rules-light approach avoiding as much number-crunching as possible with the aim of keeping action as the focus of the game - worth reading if this style of game appeals even if you are not using this setting. One intriguing idea reflects the racial melting-pot Ave Molech has become, using a system of Heritage to accommodate a character with several different races in his ancestry, rather than the easily-derived half-and-half model. There's also more background on what the deities which created the place were really doing, and the few cults which survive in the godless present day. Oh, and there are plenty of adventure ideas, some based on location for those who want to explore, and others involving the characters in intrigue or open warfare.
Next is woven in the first 4 Journals - if you buy this book, only buy Ave Molech Journals Vol.5 as the material from the others is in here! The concept is a combination of short story with adventure seeds and other game ideas. There's lots of atmospheric description as well as useful ideas - and much could be used in another setting if you prefer.
Finally, Chapter 11: A First-level Campaign is just that - a linked series of events aimed at a group new to Ave Molech (although they'd do better at 4th-6th level than 1st!). Reaching out to sieze and involve the characters from the outset, a simple plea for help from a frantic halfling whose clockwork cleaner has gone beserk leads to... well, a series of adventures which demonstrate how Ave Molech is such a unique setting to explore.
Overall, this is indeed a rich and intriguing setting with a heady blend of steampunk and fantasy with elements of horror lurking, potential for exploration and intrigue and scope for a range of adventures to suit all tastes, especially those which go beyond pure combat and loot aquisition.