Earl Spencer has marked the 29th anniversary of Princess Diana's death with poignant photos of their family home on his 'least favourite day' of the year. 

Charles, 62, the younger brother of the late Princess of Wales, made the sombre admission as he shared three pictures of the Spencer ancestral seat, Althorp House in Northamptonshire, on Instagram. 

The sobering photos showed the flag at Diana's home flying at half-mast as the Earl paid tribute to the 'People's Princess' who tragically died in a car crash in Paris on 31 August, 1997 at the age of 36, leaving behind her two sons, Prince William and Prince Harry. 

Charles's moving Instagram post comes ahead of the release of his 'historic' memoir chronicling Diana's childhood, unhappy marriage to now-King Charles and the aftermath of the crash that plunged the world into mourning. 

Announcing the news earlier this month, Earl Spencer, 62, said: 'With the 30th anniversary of Diana's tragic death on the skyline, I've been inundated yet again with requests for interviews about my sister.

'This has convinced me to write down my own thoughts and memories, once and for all, rather than having the discomfort of again reading the opinions of others, many of which are based on untruths that have become accepted over time.' 

Swan Song is expected to provide an unprecedented inside account of the seismic week during which the House of Windsor was rocked by a global wave of public grief and anger.

Its author, Earl Spencer, had a ringside seat during his sister's 15-year marriage to the future King, their eventual bitter separation and, in 1996, their divorce. 

The Earl famously delivered a raw and defiant eulogy during Diana's funeral at Westminster Abbey, which was broadcast to an astonishing worldwide audience of 2.5billion.

During the powerful oration, he highlighted how his sister 'needed no royal title to continue to generate her particular brand of magic'.

Reflecting on Diana's 'extraordinary' legacy, he added: 'It's different things to different people, particularly to women of a similar age. They really invested their lives in hers.

'Maybe they had an unhappy marriage, maybe they battled an eating disorder. There's plenty of Diana to look into and take your bit out of – almost like a horoscope, you can make it make sense for you.'

Swan Song is due to be published in the UK by Penguin Michael Joseph, a division of Penguin Random House, on September 22.

The book's striking cover features an elegant black-and-white photograph of Diana.

Publisher Daniel Bunyard today said: 'This is a book of historic importance, telling as it does the story of one of the 20th century's most iconic and culturally significant figures, but it is also utterly unique in being written by Earl Spencer, from a vantage point of such intimacy.

'Charles Spencer is both a loving brother and an acclaimed memoirist as well as a celebrated historian, and all these facets are brought together in Swan Song.

'It is extraordinarily moving, beautifully written and remarkably candid, in no small part due to Charles's rare ability to access great emotional depth without sentimentality, honest observation without rancour, and warmth without gloss.

'While these pages contain many revelations that will draw considerable attention, readers will find, for the first time, but also for all time, a fitting tribute to the beloved woman who was Princess Diana.'

Charles Spencer and Diana grew up at the idyllic Park House on Queen Elizabeth's Sandringham estate in Norfolk, until the family moved to Althorp in 1975 when Charles was 11.

Diana affectionately called her younger brother 'Carlos', while he would call her 'Duch' – short for duchess. 

The Duke and Duchess of Sussex and their children Archie and Lilibet reportedly visited the Althorp estate last month, where they are thought to have laid flowers on Diana's grave.

The princess's final resting place is on a tree-covered island in the middle of an ornamental lake called the Oval Lake.

The anniversary of Diana's death comes just days after her youngest son, the Duke of Sussex, his wife Meghan Markle and the couple's two children, Prince Archie, seven, and Princess Lilibet, five, moved back to Britain after six years in the US.