One of the greatest relationships in all of human history is surely the relationship between Shoghi Effendi, the Guardian of the Baha'i Faith, and Bahiyyih Khanum, his great-aunt, known as The Greatest Holy Leaf.
Bahiyyih Khanum, The Greatest Holy Leaf
Photograph © 2010 Baha'i National Archives, Wilmette
Used by permission
The beauty of this relationship is shown in this tribute which Shoghi Effendi wrote following her passing in 1932.
In addition, Marjory Morten wrote an essay about the Greatest Holy Leaf which was published in Volume V of The Baha'i World:
There are three excellent books about the Greatest Holy Leaf. The first is "Bahiyyih Khanum: The Greatest Holy Leaf," published by George Ronald and available from the US Baha'i Publishing Trust, and also available as a free download on the Baha'i Reference site at the Baha'i World Centre. This book contains the Writings of Baha'u'llah, Abdu'l-Baha and Shoghi Effendi about the Greatest Holy Leaf, and an extensive collection of her own letters and prayers. The second is "Prophet's Daughter: The Life and Legacy of Bahiyyih Khanum," by Janet Khan, available from the U.S. Baha'i Publishing Trust. It is a superb examination of the character and life of service of the Greatest Holy Leaf. The third is Marzieh Gail's monograph, "Khanum, the Greatest Holy Leaf," available from George Ronald. Mrs. Gail, an exceptional Baha'i author and translator (many of her translations are in the first book listed in this paragraph) had the advantage of meeting and spending time with the Greatest Holy Leaf.
Another wonderful book that includes a chapter about the Greatest Holy Leaf is Jacqueline Mehrabi's "Immortal Heroines" available from George Ronald.