Old-School Essentials is a modular game where the core rules of the game are extended by additional "rules modules" which add content for different genres of fantastic adventure. The Old-School Essentials Rules Tome is a compilation of the core rules plus four rules modules, forming a complete game for Classic Fantasy adventures:

The Referee's Tome. Cover art by Peter Mullen. Details in gold foil. 248 pages, A5 (Digest-sized). Old-School Essentials Advanced Fantasy book 2 of 2. The Referee's Tome contains a massive bestiary of over 300 monsters, extensive encounter tables, a hoard of over 300 magic items, advice on refereeing, and guidelines for creating adventures. In conjunction with the Player's Tome, everything a referee needs.