::::Hi. Ok, to be very concrete. One can not use this book as a source on the Katyn Massacre for example. For that you will need sources on the Katyn Massacre. Simple. It is not enough to cite a book, that mentions the Katyn Massacre. On the other hand one can use this book as a source on what The Black Book of Communism has to say about something. Obviously.  

 ::::That being said, it should also be taken into account that this book is biased by design, has an agenda and that some of its "facts" has even been disputed and questioned by academics and in the mass media. 

 ::::PS. Personally, I really don't understand why anyone would go to such lengths, trying to make this book something that it clearly is not. If one is keen on a specific topic, why not just find and use solid, proper sources? It can't be that difficult. You might even find some objective ones inside the book, depending on what exactly you want to write about.