Inside The Warhammer Fantasy Roleplay Companion, you'll find a detailed look at life on the Empire's Waterways, a disturbing look into the Old World Carnivals, extensive rules for trade and commerce, an overview of Old World Astrology, Old World Medicine, details on a new threat of The Cult of Illumination, an expansion of the Old World Bestiary, and more.
It's pretty difficult to actually review this, because it's essentially just a collection of random smaller articles thrown together in a single book. However, they tell you directly up front that that's what it is, so it's not like I was getting anything I didn't expect here.
There's no single unifying theme or even a pretense of one. An article about carnivals in the Empire follows one about the world outside the Empire (which isn't really more detailed than the overview in the main book, so I'm not sure why it's included). The organizations include an Empire-wide conspiracy of artists and nobles who worship Slaanesh and Tzeentch and one dwarf's gun shop--I mean, it's a nice gun shop, but it's not really in the same league. There's an extensive section on trade and rules for trading next to a brief section on what being born under the different star signs means.
It's basically impossible to provide a full review without individually reviewing all the pieces, so the most I can say is look through the table of contents and see if there's anything that catches your eye.