Why is a bestiary worth 5 stars? Quite simply, it is probably the best supplement I have ever read, for any system. It has a perfect balance of rules and background fluff.
The book is divided in two - at the back we have the alphabetical list of creatures, more than enough for games of Warhammer Roleplay. All the usual suspects are included, including animals, and there are useful sections on expanded chaos mutations and a 'monster' magic career (the Shaman) which addds some handy extra elements to your games. This part of the book is absolutley fine as a stand-alone rules-only bestiary as it gives you everything you need, exactly what you want when playing or writing an adventure.
However, the first section is where this book really shines. Here we are presented with different viewpoints on all the creatures, all grouped together under more general titles, such as Greenskins and Undead. Each creature has observations from peasant farmers (often humourous) to scholars (generally more thoughtful) and, most interestingly, themselves - which can sometimes be suprisingly insightful. In its pages you can read what ignorant villagers think of Werewolves and what professional soldiers think of Demons, not to mention what Dragons think of themselves.
This is very useful from a gaming point of view, giving the thoughts, opinions and experiences of different people across the Empire.
Overall, this book gives you exactly what you want and expect from a bestiary - and then it gives a great deal more.