This book exudes atmosphere and a sense of place and time. It's a Polish mythology and folklore based point-crawl generater - both above and below ground. The woodcut-style artwork is phenominal - I've gone flipping through the book, just looking at the artwork, many times. The book is over half bestiary with background text, a rumour table and a "what are they doing" table for almost every creature. The exception is the named godlings with descriptions, only - the book states you will not run into them accidentally - either you are searching them out, or they are searching you out. The ordering of the Bestiary is baffling - I haven't figured out if there is any order beyond random. Fortunately, it is divided into sections (People, Elementals, Godlings, etc) and every single one is in the Table of Contents. Many of the creatures are familiar, but different from the way you know them.
There is a Floriary (Arborary?), which is cool, but a bit disappointing - real world trees with the folklore beliefs around them. But you've got dragons and poleviks. Maybe there are no mythological plants in Poland? They don't hide as well as animals. The book is officially system neutral, but the stat blocks are for Knave and there is a hacked version of Knave, called Chieftan, in the appendices. Other than the addition of very unreliable and slow black-powder weapons, I didn't notice any obvious differences from Knave 1e, but it has been a while since I last read it.
Complaints - some klunky language, and the Viper has the Wolf's description - fortunately the stat block is correct. Weird Ordering of the Bestiary and no index, both mitigated by a comprehensive ToC. The book is intended to generate a point-crawl as you play. Great for solo. But the layout is not condusive to a GM doing it on the fly. The are tonnes of tables to generate almost everything, except encounters with the mythological creatures. You'll know that the caravan is staffed by astrologers from the University of X in Y, here to study the effects of the comet on the local plant growth, but only that they're being harrased by something out of the Monstrosity section of the Bestiary. So much page flipping to create the situation, then it just stops, while the players are presumably sitting in rapt attention watching the GM furiously scribble notes. The Stygian Library can pull it off because all library sections are contained and the Bestiary is short. BCG cannot. You'd be better off rolling up the session's point-crawl in advance and creating small encounter tables based on that. Worth having, and worth playing - just not quite the way the author suggests.