From publisher blurb:
The Compendium is a hundred page compilation of all the races, classes, sub-classes, feats, spells and other options published by Rats in the Rain.
It is a compendium. Everything in one place with revised rules for versatile play.
Never be stuck with boring class options as there are dozens of new ideas here for you to work with.
Maybe you have the other books and just want it all in one place. This is the book you'll need.