The family photo is on the cover of upcoming book by biographer Sally Bedell Smith

Her father called them “us four” to underline their tight family unit at the heart of the British monarchy — and that closeness has rarely been evoked as well as in a picture that’s emerged of the late Queen Elizabeth and her family.

Headed by King George VI and his wife, also Queen Elizabeth, the family was completed by Princess Elizabeth and her sister, Princess Margaret.

The four royals posed for the informal portrait in the White Drawing Room at Windsor Castle in 1943. Captured by Cecil Beaton, who took many photos of the royal family then and into the Queen’s reign, the picture was chosen for the cover of an upcoming book by Sally Bedell Smith.

The photo combines what the author calls “grandeur and tenderness” and reminds her “of an observation from one of Princess Elizabeth’s friends that they were ‘four people who mean everything to each other, whose lives form one spiritual whole, independent of the aid of all outsiders or even relations.’ ”

The book jacket cover picture of George VI and Elizabeth. RANDOM HOUSE

Bedell Smith, whose book George VI and Elizabeth is published in April, tells PEOPLE, “When this picture was taken, Princesses Elizabeth and Margaret had been busy preparing for Aladdin, their Christmas pantomime performance in the Waterloo Chamber at the castle. Among the expected guests was Prince Philip of Greece, with whom Princess Elizabeth had been in love for four years.”

The writer, who is a renowned biographer of the late Queen, her son King Charles III and Princess Diana, turned her attention to George VI and Elizabeth several years ago and was given access to the Royal Archives where their diaries and letters are held.

Princess Elizabeth, Queen Elizabeth, King George VI and Princess Margaret. NIXON & GREAVES/MIRRORPIX/GETTY

“I gained many new insights into a wonderful love story that began in 1920. I came to appreciate more deeply their commitment to duty and service, along with their courage,” she adds. “They repeatedly risked their lives as they consoled the victims of German bombing and encouraged civilians and military forces in the fight against the Nazis.”

King George VI and his daughter, Princess Elizabeth. LISA SHERIDAN/STUDIO LISA/HULTON ARCHIVE/GETTY IMAGES

They were also picking up the pieces of the monarchy in the wake of the scandal caused by the 1936 abdication of George’s brother Edward VIII, who gave up the throne to wed Wallis Simpson.

“I hope my book gives readers a new understanding of how George VI and Elizabeth rebuilt the monarchy after the abdication of Edward VIII and earned widespread admiration as they matured in their roles as King and Queen,” the author says.

King George VI and Queen Elizabeth on the day of their coronation, May 12, 1937, with Princess Margaret and Princess Elizabeth. UNIVERSAL HISTORY ARCHIVE/UNIVERSAL IMAGES GROUP VIA GETTY

Bedell Smith says she will bring fresh glimpses of their private lives “including many lighthearted moments amid the dark days of the war” — a half dozen dances in the drawing rooms at Windsor Castle that lasted into the small hours and screenings of more than 40 movies including Casablanca and Gone With the Wind.

At the time the photo was taken, Windsor Castle was “an island of serenity after four years of war, in which the monarch and his consort had shown inspiring leadership that resonates today.” (The portrait was taken in the same room where the Queen shot her comedic scene with Paddington Bear for the Platinum Jubilee in the summer.)

The Queen succeeded her father George VI when he died suddenly on February 6, 1952. Her coronation was held over a year later in June 1953. Underlining her closeness to her dad, the Queen issued a photo of them together on Father’s Day earlier this year.