This book transports you to the very heart of the Commonality of Humankind. The Core Worlds are where humankind has its origins, settled in ancient times and containing some three hundred populated worlds. This is where the most extreme parts of the Commonality philosphy are accepted as givens: many people may find this rather uncomfortable. They are tradition-bound and conservative in outlook, very stratified and detest individuality. Religion, unlicensed sexual activity and news are also outlawed as being 'disruptive'. They've been like that for a long time, but Rediscovery is now opening up Frontier worlds again - a threat to some, a glorious opportunity to others.
Yet the Core Worlds are not homogeneous of themselves. Each is unique, and they can be grouped into five regious. The first is Old Earth itself, where humankind first began. Then there's Manhome, the entire solar system in which Old Earth is found. The settled solar systems within 100 light year (LY) of Old Earth form the Old Commonality. Further out are the Penumbral Worlds and finally the Satellite Domains. Chapter 2: The Structure of the Core provides maps and information to explain all this, with notes on cultural regions and explanations of how folks travel around. On a given planet, individual vehicles are rare but mass transit systems meet most needs. Many worlds do not permit spacecraft to land, those that do generally operate an 'air traffic control' system to keep flying spacecraft in known patterns. Interstellar travel is frequent and speedy, it's generally easy to find a ship going where you wish to go. There are also 'gates' - but you start at a spaceport and get into a ship which goes through the gate... and there's loads of detail - and even the odd plot hook - about getting around the Core Worlds. Be prepared to think in three dimensions, though!
Next, Chapter 3: The People of the Core introduces the startling diversity to be found here. It might have been challenging to think of your ship as a 'person', now get to grips with planetary sentiences, as well as a vast variety of sentient beings who may be autotrophes (green and capable of photosynthesis), adapted to an extreme environment or plain weird because... well, that's the way they are. The concepts of 'government' and 'administration' are inextricably combined, producing vast bureauocracies overseen by sentiences, with departments or Instrumentalities dealing with various functions. Despite all this, factions abound, each with their own ideas... yet society as a whole can be quite stifling and hidebound. Thousand-year-old algorithms dictate the aestetics of music, education concentrates on learning how to do things the correct way and studying the great masters of the past, with innovation and experimentation regarded negatively. Unlicensed reproduction is illegal, and once born most youngsters are raised institutionally - you need a special licence to raise a juvenile in the parental home. Life is communal in general, very structured, yet with great opportunities and resources made available to all. Deviance from societal norms is not acceptable - and visitors have to learn them fast as little allowance is made for them. Deviants can expect to be re-educated.
Then there are chapters on each of the five regions beginning with Old Earth itself. Here the majority live in arcologies, and this concept is explored in detail. Oh, and the moon has been terraformed into Green Moon. It's all quite different from what we are used to... Next is Manhome, the rest of the solar system, then on to the Old Commonality and beyond. Each is described in rich and evocative detail, it's easy to imagine every place that is written about. Plot ideas abound, so wherever the party ends up, there ought to be plenty going on around them - but reading the paragraph or two about each planet spawns plenty of your own as well.
Finally, Chapter 9: Gaming in the Core Worlds provides a wealth of information that will give you plenty to think about if you intend to run games set here. Several themes are suggested, each with a series of questions that your plot may be attempting to answer, ample scope for exploring and discovering, and perhaps - if outsiders - getting into trouble.
It's quite a breathless sweep across a vast cube of space that has Old Earth at its centre. It tells you a lot, yet if you want to actually use it in your game you will need to put in the work to create plot and antagonists and all the other resources required... yet here is a magnificent framework on which to build.