From the publisher:
Perdition is a third wave Old School Renaissance “Adventure Role-Playing Game” clone. It is a heavily modified clone in the spirit of the Basic/Expert set, with some elements and terminology from later editions. It is a third wave clone because it is not a game designed to directly emulate or provide an improved version of the “Fantasy Adventure Role-Playing Game”. It is not a game designed to allow you to create your own fantasy realm and have whatever type of adventures you wish.
It is a game designed to allow you to explore the world of Perdition. It crosses the line of setting books that work with whatever ruleset you are using and provides the setting information via mechanics, classes, equipment, spells, and monsters, instead of through large blocks of flavor text and fiction writing. It is a game that defines a setting. It is something Game Masters have been doing for years made explicit.
Make no mistake, this is a version of the classic fantasy adventure role-playing game. You may feel free to loot Perdition for house rules, game ideas, classes, and more! You still roll up characters and explore dungeons and hexes. You still kill monsters and recover treasure. Perdition assumes you already know how to play “Adventure Role-Playing Games” and are seeking an idiosyncratic experience of adventuring in a specific Game Master’s world.
The hardcover has the following text:
"This hardcover contains an additional illustration of the animals of Perdition, as well as some slightly expanded content."
It doesn't state what the expanded content is, or which other version of Perdition is expanded upon.