Momigliano writes in his essay A Piedmontese View of the History of Ideas:
Lord Acton managed to become famous for a book on liberty he did not write.
What is meant by this puzzling sentence?
The quote refers to two things about Lord Acton. First, he was anything but prolific as an author:
He is notorious for having rising to the heights of the historical profession without actually writing a book; the only work published in book form during his lifetime was his inaugural lecture when he became Regius professor of history at Cambridge.1
Second (and somewhat related to the first), he was obsessed with writing the definitive "history of liberty" but insisted on sourcing and researching it himself. The following makes use of a similar phrase, but puts it into a little more context:
The Cambridge historian Lord Acton (1834-1902) – who is best remembered for his maxim “Power tends to corrupt, and absolute power corrupts absolutely” – spent much of his professional life working on a single book, a comprehensive history of liberty. As one of the most knowledgeable historians of the nineteenth century, Acton was uniquely qualified for this ambitious project. Unfortunately, however, his rigorous standards of scholarship eventually got the better of him.
Distrustful of secondary accounts, Acton insisted on doing original research in every phase of his history. And since his history spanned over 2500 years, covered many different countries, and dealt with everything from the history of religion and philosophy to the history of political movements, it became virtually impossible for any one person, however brilliant and industrious, to complete the task that Acton had set for himself. Consequently, Acton’s masterpiece was never finished, and his history of liberty, the work of a lifetime, became known as “the greatest book never written.”
Much of the work he did on The History of Liberty has been published in various essays and collections, but his opus was never completed.
1 Boyd, Kelly. Encyclopedia of Historians and Historical Writing, Volume 1, p. 1
2 Smith, George H. Lord Acton and the History of Liberty, Part 1, online
The only hit I could find for misattributions involving Lord Acton is for a simple statement, not an entire book. This is what is sometimes today called Acton's Law: "Power corrupts. Absolute power corrupts absolutely".
Its true that isn't quite what he said. The full quote, from a letter he wrote to fellow historian Bishop Mandell Creighton in 1887 is:
Power tends to corrupt, and absolute power corrupts absolutely.
The popular phrasing only alters the sentence slightly, in the laudable interests of brevity and clarity. However, many argue that without the "tends to" it turns a statement about pressures into a assertion of an unstoppable mechanism.