This Worldbook will be updated over time, but its a concept I've started up on the RPOL site.
Here are the current basics:
Game Concept:
The over all game concept is that players come from a world that is a nexus between worlds (refer to Game: Home Plane). For the most part, players will most likely create characters which are adventurers in one way or another and seek to explore worlds and planes outside the Home Plane. Some ideas which might lead up to this are players who are members of guilds going on quests to gain prestige in those guilds, players who are part of temples searching for holy relics, characters who dream of fame as bounty hunters and owning a mansion on the most luxurious of the residential islands, or maybe a merchant cook who likes making their food out of the most bizarre ingredients… Either way, access to the Portals leading to these worlds and planes is monitored via a Pass Booklet (as is access to the island preserves). Obtaining access to higher level planes or islands depends on quests, fame, and if the character can afford paying the fee for the right pass. Of course, like any government controlled good or service, illegal portals are out there to be found and used, and the occasional rogue portal can be discovered.
The Home Plane consists entirely of one one kingdom: the floating kingdom of Astria. Astria is not actually one land mass, but a series of floating landmasses, connected together by mystical chains and giant bridges, said to have been forged by ancient gods long since passed. Astria floats through the clouds against the backdrop of a pale blue sky and shares the heavens with fluffy white clouds and a glorious yellow sun. There are also two moons, one called the Mother and the other the Daughter, and a sea of stars which have the odd habit of completely rearranging themselves every few months, making astronomy a lifetime hobby. It is rumored that there is an ancient world far below Astria, but no one has ever gone for enough below in order to discover if this is true, and if they have gone below (by building gliders and other crazy contraptions), they have never returned.

There are a score of such floating islands that make up Astria. However, most of the islands are undeveloped and remain full of lush vegetation, flowing waterfalls (the water on the islands mysteriously bubbles up to the surface then flows off the edge of the islands), animals, and even monsters (which many Astrians hunt for training and prestige). Approximately 13 of the 20 islands are undeveloped and even protected by the Conclave of Guilds. In order to travel to these islands, one needs to have authorized entry stamped into their Pass Booklet. The other 7 islands are populated and divided up between residential, commerce, government, and religion. The residential islands make take up 4 of the 7, and who lives on which island is entirely determined by income and wealth. IF one has the wealth and means, one can always buy a new home on one of the upper scale islands. Of course, wealth generally is obtained by adventuring, quests, and exploring, though sometimes it can be achieved through government and commerce.
For those who have no homes, generally the sign up for either a Church or a Guild (though the lowest residential island has safe homes), and live within their respective temple or guildhouse. The island for commerce holds all the main business thoroughfares, guildhouses, and merchant stores. The island for religion has many temples dedicated to ancient beings known as Espers (often Summoners and Callers are associated with these temples, calling upon the Aspects of the Espers). The island for government is actually a giant floating mountain. As one ascends the mountain, they can access different Gates to go World Jumping (if they have access with their Pass Booklets). It is here that the Conclave rules, a council formed of four Guild Heads, representing the Warrior's Guild, Mage's Guild, Adept's Guild, and Expert's Guild. Although these four guilds are the official ones represented in the commerce sector, there are rumors that subguilds can be found.
Welcome to Astria…. enjoy your stay!