The central thesis of this book is that the 'secret history' and rumours of the murder of James I by Buckingham/Charles had a much more profound impact on the political history of the seventeenth century than previous scholars have credited it with. They have looked at libels and some political documents to provide evidence for this view, drawing in aspects of the cultural turn and post-modernist history, to construct a narrative that is interesting if not compelling.
Aside from the fact that post-modernism is not my bag, I have a number of issues with the argument. Firstly, the use of evidence is overly selective - there is no analysis of the wider documentation or context of popular history in the early- to mid-seventeenth century: some rough percentages of the print and manuscript cultures would show how important the libels, etc, actually were in the general scheme of things, i.e. what proportion mentioned the 'murder'. Quantity of material instead seems to be glossed over by quoting ad infinitum from particular tracts, which, at best, seems unnecessary and, after a while, rather dull. It seems almost as if they are practising the psychological technique of looking for everything of a certain colour in a room in order to say how much of that colour there is in the world, instead of taking in the wider picture. There is also a reasonable amount of what I can only describe as grasping at straws: there is a distortion of certain events and fact. For instance, it seems rather poor to say that the rumours played a central part in the trial of Charles when they weren't even mentioned in the charges (ch. 21). The authors have tried to explain it away but I can see no other reason for excluding them than that the Rump knew they were on dodgy ground. Likewise, tying rumours into the awful time Charles II had in Scotland in 1650-1 is not really feasible. Charles II was made to renounce both his mother and father as there was no other way, psychologically or politically, that the Scottish could countenance taking up arms against one king while supporting his son.
As a mark of respect to Cogswell and Bellany, I will assume that they have overplayed the rumours as a rhetorical device to draw attention to the fact that some people believed Buckingham and/or Charles capable of murder and that it therefore deserves inclusion within the overall 'high road to civil war'.