From the publisher's website...
This is the book we should have released second. It is a 296 page comprehensive guide to the Dying Earth.
It includes reliable information on everything that has ever existed. (This is not strictly true.) It details all the major people, places, creatures and phenomena within the Dying Earth and also offer the expansions, speculations and comments of other contributors, as well as asides into the Vancean oeuvre.
For the Dying Earth roleplaying game, the Compendium includes more than two hundred adventure seeds, game statistics for eighty creatures, more than a dozen spells and cross references to all our other publications. The Compendium is lavishly illustrated, with beautiful maps.
When you are not poring over its pages, its sturdy frame will add character to your bookshelf.