Shapiro looks in great detail at the state of London and England three years into the reign of the new Scottish king, and how this can be demonstrated to have affected Shakespeare's choice of material and approach to King Lear, Macbeth and Antony and Cleopatra. Chapters of historical scene-setting, on politics, religion, and the economics of the theatre, more or less alternate with chapters about Shakespeare.
Of the three plays, I know only Macbeth well, or thought I did. Of course, a play that starts and ends with the off-stage killings of two Scottish kings has to be seen as a reaction to the Gunpowder Plot the previous year; but Shapiro very impressively threads together Shakespeare's own Warwickwhire connections to the plotters. He also looks at the play's links to witchcraft, including King James' own writings on the subject, and its reflection of the moral panic around "equivocation", the 1606 equivalent of worrying about teenagers running away to join ISIS.
His strongest section, however, is the first chunk about King Lear as a reaction to James' plans to unify Scotland and England (which did not become formal for another century) and also as a reflection of Shakespeare's own economic independence and ability to pursue new dramatic paths, though rooted firmly in his own immediate artistic environment. The division of Britain (ie England and Scotland combined) is a fundamental error which King James is now, by implication, planning to overcome. King Lear is just one of many Shakespeare works to rip off an earlier work by someone else, but in this case he took much more liberty with the plot, in particular giving it a tragic ending. Shapiro convinced me to go and give King Lear another try.
I was less convinced that there is all that much that is interesting to say about Antony and Cleopatra. Shapiro makes a more than valiant effort, looking at the complex politics around marriage in the Jacobean court and also at how Shakespeare tended to write sequels almost immediately, rather than leave them several years as he did in this case (if you consider Antony and Cleopatra a sequel to Julius Cæsar). But basically, it's a less engaging play than the other two.
Shapiro's core case is that we neglect the reign of James VI and I unfairly, as a footnote between the Virgin Queen and the Civil War. There was a lot going on in England in the years after 1603, and that includes some of the greatest works of England's greatest writer. I'm convinced.