From the introduction:
The “BBOK” is many things. It’s a collection of giant monsters culled from the annuls of movie history. Any fan of the genre will be able to recognize a favorite monster here. It is also a game master aid for the “Mecha vs. Kaiju” game, offering statblocks for adversaries to throw against players. And finally it is an aid to players of “Mecha vs. Kaiju,” giving them an insiders look at how citizens of Japan and the world view the creatures that daily menace their lives.
This supplement is written like the introduction to the real BBOK, and features sections on “Kaiju Biology” written by biologist Merle Clark, as well as biographies of the writers and annotators of the BBOK. Everything in this supplement is free to be read by both players and game masters.