This fictional book was mentioned in practically all of the installments of the main series. Now you can read it. As the title says it is a Bestiality: notes on all the monsters the Spook and his colleagues had to face in their line of work. For them knowledge is the power, so the Spook and his apprentices keep this book updated. As such it is full of short stories about some of his dealings with different monsters; these were only briefly mentioned in the main plotline.
The only reason I read this book is that is was available in the local library. This and that is was short. It actually turns out slightly better than I expected. The short stories I mentioned filled up some unexplained parts of the main story quite nicely.
The illustrations were equally good; I would not recommend looking at some of them at night if you are alone. As an example, take a look at one of the most harmless creature of the book - a banshee:
It is a beaut, is not it?
On the negative side some of the monster descriptions were quoted in the main books verbatim, so I remember reading most of them before. Still, the Bestiaity is good enough and short enough to warrant 3 stars.