From the back of the book:
The Second World Sourcebook bridges the gap between modern day adventuring and traditional fantasy roleplaying. It provides a "campaign template" for running a dual-world game. By combining the sourcebook with your own favorite setting you can take your fantasy characters and run them through a modern day scenario, or take your modern characters and run them through one of the many fantasy modules currently available. Most modern settings already describe the impact of fantasy elements on them and include rules for handling magic. But what would happen if people from the modern world regularly trafficked with those in a more traditional fantasy world? What impact would it have if powerful wizards originally studied computer science at UCLA? What would the social institutions look like if the progress of ideas and politics we've seen in our world carried over to a place where electricity and gunpowder simply didn't work, but magic and divine power did?
The Sourcebook supports both the d20 Modern Rules and the standard d20 rules and provides a complete set of genre and technology rules for customizing the way reality works in the two worlds, in alternate planes, or even in your own game world. Over 200 powers spread across 8 prestige classes (with an alternate, feat-based system)are detailed. Each class gets to pick one ability per level from a list of at least 25 special powers; this provides maximum customizability in character development. It also includes a complete set of influence rules with over 30 organizations and 100 favors to request; this system provides a nifty method for adding crunch to the political aspect of a game. The Second World Sourcebook also provides a basic campaign setting designed to show off the full potential of genre jumping.