The political constitution
J.A.G. Griffith, The Political Constitution (1979), the essay that named the idea. Richard Bellamy, Political Constitutionalism (2007), the modern theory. Adam Tomkins and Jeremy Waldron, who argue that contested questions belong to elected legislatures. See the political constitution.
The legal constitution
Tom Bingham, The Rule of Law (2010), the classic defence of the courts' role. Ronald Dworkin on rights as trumps, and T.R.S. Allan on a rights-based common law. The other side of the argument, put well. See the legal constitution.
History and the old constitution
John Fortescue, The Governance of England (c.1471). A.V. Dicey, The Law of the Constitution (1885). Walter Bagehot, The English Constitution (1867). Alan Macfarlane, The Origins of English Individualism (1978). The roots Starkey draws on, set out on the English constitution.
The wider diagnosis
Jonathan Sumption, Trials of the State, from his 2019 Reith Lectures. Vernon Bogdanor, The New British Constitution (2009). David Goodhart, The Road to Somewhere (2017). Independent voices who reached related conclusions, on not just Starkey.
Starkey himself
David Starkey's own case is made across many lectures and interviews rather than a single book. They are collected, grouped by theme, on the sources page.