Renegade Crowns is a toolbox for designing and developing principalities in the tempestuous land of the Border Princes. Inside this sourcebook, you'll find extensive guidlines for creating the lands and the secrets they hold, mechanics for creating princes, and a complete system for populating your region with Greenskins, Chaos, Undead, and other monstrous horrors. A fully detailed region to kick-off your own Renegade Crowns adventures is also provided.
This book is quite good at what it does, but what it sets out to do is definitely not for everyone.
This is not so much a place description like Realm of the Ice Queen or Knights of the Grail is. It's much more of a toolkit. It's designed so that the GM can build principalities instead of providing a list of those available. In the chaotic borderlands, it's perfectly possible to ride into a flyspeck town, go in to the only tavern, get into a bar fight and stab someone in the throat, and find out you kill the local prince and are now the new rulers of Mudville. The book says that because rulers change so easily, any listing of principalities would be pointless because they'd be out of date by the time the game began. Because of that, the books sticks to providing more general advice about building a principality and the kind of challenges the players face while ruling it.
And in the end, that's the main problem. Renegade Crowns does not have anything tying it specifically to the Warhammer world. Sure, there's the charts in the first chapter, and the list of names of the back of the book, but that's all. If you ran a search-and-replace for Arabyan or dwarf and replace them with the appropriate listings from the fantasy world of your preference, you'd be able to use this just as well for any other fantasy world. This is not automatically a bad thing, as it means you get more use out of it, but it also means that you could use the other books published to create fantasy kingdoms for player characters in other settings just as well take care Warhammer principalities. For example, the rules to deal with internal and external troubles to your principality specifically do not have any kind of game stats other than the scores to determine when internal or external troubles occur. This is good, because it lets the GM easily adapt to the situation. However, it also means you can port the same system to any other game, or import a system from another game just as easily.
So, is useful in general as it is, I can't give it more than three stars. There's nothing here that you can't find in any other similar book for another system, and if you were looking for a canonical list of the principalities in the borderlands, forget it. The only example is the example given for building one. While the humor displayed throughout is classic Warhammer, especially in how much of the land is crap, people fight each other all the time, princes need to display extreme brutality and generosity is for fools, etc., that's more an issue of tone and does not affect the information. It does make for a fun read, but it doesn't make it any more or less useful for any other game system.
For a certain kind of game, Renegade Crowns is excellent. However, there are plenty of other books that do the job just as well, and if you already own one, there's no reason to get this.