To my knowledge this is the only letter that Sant Baba Nand Singh Ji wrote. (It was dictated to Sant Baba Kartaar Singh Ji who transcribed it)
“Waheguru Ji Ka Khalsa Waheguru Ji Ki Fateh
Respected Mother and Father,
I am quite well here with the blessings of Dhan Dhan Satguru Sri Guru Nanak Dev Ji Maharaj. I hope all of you are in the best of health. My dear mother, you kept me in your womb for nine months and then gave birth to me. Many times in the severe winter season of December and January I slept comfortably while you had to sleep on the wet side of the bed. You suffered many discomforts and troubles due to me while bringing me up. You had a desire that I would earn a lot of money for you when I grow up. I failed to fulfil your desire and I did not earn any money for you even though you brought me up facing so many problems. You sent me to such a wonderful place, which is denied even to those who have done a lot of meditation and prayers. How can I repay you for this kindness? As you sent me to a place like that and on account of the blessings of Satguru Sri Guru Granth Sahib Ji Maharaj no one will even ask you for the accounts of your deeds. All of you will go straight to the laps of Dhan Guru Nanak Dev Ji Maharaj where you will rest for eternity.
Waheguru Ji Ka Khalsa Waheguru Ji Ki Fateh”
Waheguru Ji Ka Khalsa Waheguru Ji Ki Fateh