Again,Harters,we rely on books/links/relatives,so how would we know ? Or they know,if it comes to that. However,perhaps it should have been more elaborated and brought in to question,like the silencer,bike and wetsuit. 
Books are improper to cited for evidence. Statements by witnesses and books are 2 different animals. Books are mainly useful to provide some background and to faciliate further research. If you cite a claim for a book you have to make clear it is a claim for a book and not corroborated.
Books are most useful when the footnotes point out the genesis of the claims they are making. If they are citing a document (including astatement) you want to read the document in question to see if their claim is true. If the claim is true you cite that document in the future not the book. When you are stuck citing the book it means you don't know if the claim is true or not. So when you cite a book you are making a allegation effectively not presenting a documented fact.
many authoris include not footnotes and provide no basis for their allegations. Those who do so at least enable you to try to check their claims out if you want to go through the effort which in some cases is a lot of effort depending on how hard it is to get a hold of the referenced materal. Plenty of times footnotes do not hold up and the referenced material do not support the proposition claimed.
If you don't check then in that case you say such and such author claims A,B,c but you don't know if it is actually true because you did not check. That is done so you don't end up with egg on your face in the event he intentionally distorted or simply made an error.
