The subtitle to this book in Pathfinder's Campaign Setting line immediately sums up the essence of Irrisen: it's a place of perpetual winter. Cursed by the witch Baba Yaga, Irrisen is a foreboding land ruled by witch-queens placed upon the throne by Baba Yaga herself and even the cities are full of dangerous, intelligent monsters like winter wolves, frost giants, ice trolls, and more. The peasantry are governed with strict rules and prohibited from leaving Irrisen for more inviting places, and even merchants and adventurers who travel to Irrisen from elsewhere do so at their own risk. Irrisen isn't an easy place to live, because if the weather doesn't kill you the inhabitants very well might. Although released to support the Reign of Winter adventure path (which takes place in Irrisen), this book works very well as a standalone source-book for GMs wanting to set other campaigns in the nation. The inside front cover of this 64-page book features a map of Irrisen, showing how it's been divided into six different provinces, each ruled by a different winter witch Jadwiga (descendant of Baba Yaga and of Irrisen's first queen).
The first six pages of the book give an introduction to Irrisen and a history of its centuries-long rule by the winter witches. Unlike some histories of fictional lands, this one is quite interesting and has on-going resonance for the "present day" state of the setting.
Next, the bulk of the book is devoted to a full summary of each of the six provinces, including their ruler, major cities, and other locations of note. Each province receive six pages of coverage, and includes a map of the province, a map of the province's capital, and a picture of a notable NPC. I won't go through the six provinces as they all share a common overall theme, but I will say I very much appreciated that a lot of work was put into making the descriptions full of adventuring hooks for a GM to exploit. Sometimes even the best GMs need inspiration, and it's much more interesting to read a source-book like this if its relevance to actual game-play is clear from the outset.
After the provinces, a six-page "Plots and Perils" section provides more detail on some particular places for adventure and potential storylines to involve the PCs. Using any of these would still require a lot of work by a GM, but there's enough to plant the seeds for several adventures. There's also a few new afflictions (curses and diseases) and some supernatural weather hazards.
Last, there's an extensive bestiary that includes random encounter tables for each of the provinces. I like random encounter tables, but what's missing from these is how often an encounter should take place. In addition, the vast range of CRs on the very same table (one table ranges from CR 2 to CR 17) are such that actually randomly rolling up encounters is likely to result in something either laughably easy or an instant TPK. This reduces the usefulness of the tables substantially, though they can still serve as a list of possible threats for a GM to pick from. The rest of the bestiary is quite solid. One of the really interesting features of Irrisen is that every century, Baba Yaga returns to take the current queen with her and installs a new queen on the throne; this occurrence is presaged by the appearance of the Three Riders, who pass judgment on Irrisen's inhabitants, from the lowliest peasants to the most wealthy winter witch. Each of the Three Riders receives a full page entry. After this, a new template ("Boreal") is created to make it easy to add winter flavour to any ordinary monster, and example stat-blocks of Boreal wolves, Boreal manticores, and Boreal annis hags are provided. Other new monsters include the guardians of Irrisen's borders (Sentinel Huts and Guardian Dolls), a dragon called a Khala, a giant called a Ved, and NPC stat blocks for a standard winter witch baroness and a winter witch "Cold Sister" (the land's inquisitors). Of the campaign setting books I've read so far, this one has by far the best and most useful bestiary.
The artwork in Irrisen, Land of Eternal Winter is uniformly excellent. Although I think I would have preferred to see more variation from province to province, I could definitely imagine setting an entire campaign in Irrisen and using this book as my primary resource for dozens of sessions of adventure. When your players have had it too easy for too long, send their characters to Irrisen--that should shake things up!