Charles Spencer, Princess Diana's brother, published his memoir "Swan Song: Diana, My Sister" on Tuesday, released by Penguin. By the end of that same day he was already apologising - and Piers Morgan was announcing a lawsuit.

The dispute is over a single paragraph. In the book, Spencer claims that in 1993 Morgan and another Mirror journalist published photographs of Diana taken with a hidden camera. The problem is that Morgan only joined the Daily Mirror in October 1995 - two years later. That is not a nuance of interpretation. It is a date anyone can verify in a minute.

Morgan responded on X: he will bring legal action against Earl Spencer and his publishers "for the demonstrable lies he has written about me". He is demanding the removal of the disputed passages and damages, and called the book's description of him - Spencer describes him as "scowling" during a phone call about a mix-up over a funeral invitation - contemptible.

From Spencer's side came a statement worth reading carefully: "As a historian, accuracy matters to Earl Spencer. He therefore wishes to clarify that his assumption was mistaken." An assumption. In a book sold as the testimony of the man who was closest to the story.

This is where the question the publishing industry does not want to open up lies. Penguin is not a small publisher without resources. A memoir by Princess Diana's brother goes through legal review before printing - that is standard procedure. How did a date verifiable with a single search get through?

The answer is probably simpler than we care to admit: the book was written to sell, and doubting your own text slows sales down. When the author is a historian, that weighs even heavier - the title serves precisely as a guarantee that somebody checked.

It will all end with a footnote in the second edition and a few weeks of tabloid coverage, which will do the book more good than harm. And the reader who bought it on Tuesday is left with a version in which a man is accused of something he could not have done.