UNDATED (CBS) -- This huge archive of letters, papers and photos reveals new details about Indian leader Mahatma Gandhi and his time in South Africa.

The documents, to be auctioned in London, belonged to Hermann Kallenbach who became arguably Gandhi's closest friend, according to Sotheby's Dr Gabriel Heaton.

Heaton says, "This is a tremendous and very rich collection that contains material that belonged to Gandhi's very close friend Hermann Kallenbach. It includes letters by Gandhi to Kallenbach and many letters from Gandhi's sons and also sons to Kallenbach - very intimate and personal letters covering many decades actually."

Kallenbach, a German-born Jew, met Gandhi in South Africa, where the Indian leader spent more than 20 years of his life before returning to India permanently in 1915.

In the letters Gandhi addressed his friend as 'my dear lower house', signing off 'with love, upper house'.

A recent book by Pulitzer Prize winning author Joseph Lelyveld alluded to a very intimate relationship.

But experts say these documents don't support this idea. Heaton says, "There is nothing in this archive that substantiates some of the sensationalist claims that have been made about the relationship. What this does show is that this was a tremendously important relationship for the development of both men."

Among the most illuminating documents are the dozens of letters written by Gandhi's sons providing details of his life in India.

"Father is becoming more and more awful," read one incomplete letter probably written by Harilal, his eldest.

This collection sheds fascinating new light on a man revered in India and fondly known as the father of the nation.

It's also expected to fetch up to $1.1 million USD when it's auctioned in July.