Sigmar’s Heirs (subtitled A Guide to the Empire) is… just that. A guide to the Empire, a gazetteer of sorts with some general history write-ups. While it may seem like a thin book, it actually contains quite a bit of stuff due to a small font (verging on “too small”) and layout with very little unused white space. There’s quite a bit of raw information here. Of course, it is a gazetteer for the most part, and as such is somewhat of a dry read. On the other hand, for many GMs this book will be used as a regional resource and not something that they read through from cover to cover, so that’s not a huge issue. There are entertaining bits here and there, and it’s quite well-written as a whole.
The book details each region of the Empire, including major cities, general characteristics of the inhabitants, and other notable information. There aren’t many detail maps here, people looking for maps of cities (for example) will need to look in other books. This is a very “bird’s eye view” of everything.
It’s a great resource for the Warhammer (fantasy) game world, and since it contains very little crunch it works just fine for other editions of the game. Notably, 3rd edition has less pagecount for stuff like this, so this book would make a good background reference even for people running the latest ruleset. My only complaint is the font size – it’s a bit too small for truly comfortable reading, at least with my aging eyesight (though lighting etc will have a huge effect, there).