My discovery of Henry Neville's Book Catalog offers us an entirely new way to examine Shakespeare sources. Instead of just looking at the plays and poems and trying to guess which sources might have been consulted, now we can look at a library of books and see which correspond with the ideas expressed in Shakespeare's works.
I want to highlight three entries in the library catalog, all of which have dependently been identified as possible Shakespeare sources: Machiavelli's The Prince, Jean Bodin's Les Six livres de la République (The Six Books of the Republic) and Giovanni Botero's Della ragion di Stato (The Reason of State).
Machiavelli's The Prince
Henry Neville had a 1551 edition of The Prince, here is the USTC Entry.
Here is the title page for a very similar edition you can view online at Google Books:
Alencon! that notorious Machiavel!
It dies, an if it had a thousand lives. - Henry VI, Part I, 5.4
Change shapes with Proteus for advantages,
And set the murderous Machiavel to school. - Henry VI, Part III, 3.2
Peace, I say! hear mine host of the Garter. Am I
politic? am I subtle? am I a Machiavel? - Merry Wives of Windsor, 3.1
There is a vast literature on Shakespeare and Machiavelli. Here is an article from the British Library on Machiavelli and Richard III. Hugh Grady's Shakespeare, Machiavelli, and Montaigne deals with these issues in-depth. Here is evidence from Richard II showing Shakespeare likely read Machiavelli. I could offer dozens more references.
Jean Bodin's The Six Books of the Republic
Henry Neville had a 1579 copy of Jean Bodin's Les six livres de la republique, here is the USTC Entry.
For discussion of Shakespeare's response to Bodin, see Campana's The Child’s Two Bodies: Shakespeare, Sovereignty, and the End of Succession, Lopez's Trespass and Forgiveness in William Shakespeare’s King Lear. DiMatteo's Shakespeare and the Public Discourse of Sovereignty: “Reason of State” in Hamlet, and Hadfield's Shakespeare and Renaissance Politics.
These references to Bodin take on a new meaning if we consider the possibility that the author of the works of Shakespeare owned a copy of The Six Books of the Republic. This is why Shakespeare Authorship Research is so important for Shakespeare scholarship.
Giovanni Botero's The Reason of State
Henry Neville had a 1589 copy of Botero's The Reason of State. Here is the USTC Entry.
You can view this 1589 edition on Archive.org.
See the Oxford Handbook of the Age of Shakespeare for a discussion of this book and Shakespeare. This article by Bragantini suggests that Measure for Measure may demonstrates a first-hand knowledge of Botero.
Summary
The discovery of this library catalog opens up many new possibilities for Shakespeare research. Up until now, it has been assumed that the author of the works of Shakespeare could not read Italian and French. Now we have strong evidence that not only could he read Italian and French, there are specific books that he likely read. This creates a whole new line of research in Shakespeare studies and validates the many scholars who have already noticed strong correspondences between Shakespeare and these works.