The Group Six tales take place on the planet Lurra, an Earth-like world with a crucial difference: on Lurra, Magic is a very real force which the inhabitants can learn to use. But the Magic of Lurra does not exist through mere happenstance. There is a scientific reason behind it all, one that is gradually revealed in the series of Group Six books.
Lurra is inhabited by two races: the ancient Eryndi and the more recent Macai. The Eryndi are physically smaller and have distinctive pointed chins and conical ears. More importantly, their lifespans extend into the hundreds of years. The Macai are similar to humans in both appearance and lifespan.
When the two races first encountered each other, the Eryndi regarded the Macai as savages and enslaved them as beasts of burden. That era is far in the past, and the two races now share the world as equals. In fact, although the Macai are the younger race, they have become far more numerous than the Eryndi.
Lurra is also home to a smaller number of entities known as spirits. Highly Magical in nature, they are mostly non-corporeal, although some of them choose to take on physical forms which can range from small enough to hold in your hand to the size of a large building.