Charles Spencer's new memoir of his sister, the late Diana, Princess of Wales, tells us "a little about Diana and a great deal about Charles Spencer – perhaps more than we needed to know," according to a Guardian review published on 22 September.

The book, titled Swan Song, runs to 400 pages. The reviewer describes it as "a fascinating document" but one that leaves the reader with "a head-spinning story of the world that created Diana and a certain sourness at the manner in which it is told."

Spencer, the 9th Earl Spencer, sold serialisation rights to the Daily Mail. Last week the newspaper led with the memoir's most explosive claim: that shortly after Diana's death in 1997, King Charles – then the Prince of Wales – insulted her in a phone call to Spencer. Spencer quotes the call directly: "'Rest assured,' he hissed, 'we'll forget her soon enough.'"

The Guardian reviewer treats that quotation as the book's fulcrum. "Your take on this quote will likely dictate how you receive the book," the review states, before offering its own verdict: "I'm willing to believe the king behaved badly, but 'hissed' is a red flag, one of those embellishments, like 'snarled' or 'flounced', that writers tend to use to make those they're writing about appear worse."

The review identifies a deeper contradiction at the heart of the memoir. Spencer sold the serialisation to the Daily Mail even though the book itself contains a line describing tabloid journalists as "those who inhabit the murky swamps of malevolence, and who need to suck their victims dry of their souls." The reviewer's dry aside: "Perhaps he meant a different set of hacks."

The central critical charge is one of self-regard. Amid what the reviewer calls "genuinely touching passages about his sister," Spencer repeatedly positions himself and his grievances at the centre of the story. The memoir, in the reviewer's summary, delivers "score settling, chest thumping, wild indiscretion and some truly touching passages about his late sister."

The review concludes with a blunt assessment of the earl's reliability as his own narrator. "Spencer, always the injured party, has so many axes grinding away in this book that at times it's impossible to hear what he's saying."

Swan Song now enters a crowded field of Diana literature, published nearly three decades after her death. Its commercial moment is already assured by the Daily Mail serialisation. Whether it adds to the historical record of Diana's life, or chiefly to the documentable record of her brother's resentments, is the question the Guardian review leaves open.