This is a couple dozen pages' worth of commentary on the Grand Stair, the liminal space that links worlds in the setting. The bulk of it is the correspondence and lecture notes of a particularly talented traveler and observer. Rob Donoghue does a good job capturing the voice of a smart, capable jerk here - Lucien is what a lot of net flamers fancy themselves, but with the accomplishments to justify a good measure of self-confidence. There are also some very nifty new sorcery spells, suggestions for new uses of existing cantrips and such, and a very sensible framework for allowing characters to develop individual aspects of the Grand Stair powers.

All of this does the most important thing a setting book like this can do: it opens up possibilities rather than closing them off. Every statement about "this is how the Grand Stair works" comes with an acknowledgement of the variety innate in a basically infinite setting, and every speculation about why the Grand Stair works like this comes with some frank acknowledgement of the limits to anyone's knowledge. If you want to establish the essence of the Grand Stair as an unsolveable mystery, this book has your back. But it also supports you if you want to use one of the theories presented here or invent some of your own. It's useful.

Highly recommended.