The basic argument of this book is that the Dutch and Spanish took seriously the things that they said about sovereignty and religion. It might seem obvious, but there are always materialist historians (often, but not always, Marxists) who want to reduce everything to economic interests; in other words, they argue that talk about non-materialist things such as religion is just a smokescreen to cover the fact that a person really wants what will help him the most financially. It is true that in politics and diplomacy, perhaps more than in any other fields, one should be sceptical of a speaker’s motives and whether they correspond closely to what he claims. But this is more the case for public pronouncements; when people make the same arguments in private correspondence or in council, it is much more likely that they are saying what they mean. Materialists (such as Jonathan I. Israel, one of the foils of this book) rarely seem to care much if there is actual evidence that people think exclusively in material terms; it is, rather, assumed, and any coincidence of actions with material gain is adduced as evidence.
The book is organized thematically, with chapters entitled “Rebels,” “Tyrants,” and “Authority.” It chiefly addresses political thought around the question of when it is acceptable (if ever) to rebel. This was a big issue in the 17th century, when people took the Bible’s admonition to obey authority very seriously. Protestants in particular grappled with how to deal with rulers who wanted to extirpate their beliefs without turning themselves into outlaws. Baena also considers the situation of Catholics who rebelled against Spain, such as those in Catalonia and Portugal.
The book has relatively little on the actual negotiations, which is reasonable considering its scope. Scholars will mostly be familiar with the sorts of arguments Manzano Baena finds in the pamphleteers and statesmen of Spain and the Dutch Republic. For casual readers, I think the book is approachable enough, provided you skip the introduction, which exists mainly to justify the book and put it in its historiographical context (presumably it was originally a dissertation).