Veer Ji check your private messages. Also, there is a book that has Hukumnamas of Sri Guru Tegh Bahadur Sahib in which Guru Sahib wrote to sangat in different places. Its a great read it shows how much of a great military planner and organizer Guru Sahib was. If you look for it on the net im sure you can find it.
There are couple of letters written by Bhai Sahib to his companions that are out of this world. I believe one such letter written to Professor Thakur Singh is published in "Purab Janam ke Milay Sanjogi". What an amazing outpouring of love it is. Professor Thakur Singh showed that letter to his collegue who expressed strong desire to meet Bhai Sahib.
Another good example of letters are the letters sent by Bhai Vir Singh jee to his friends and companions. These letters contain some very sound spiritual lessons and have been published in the form of a book.
ਸਤਿਗੁਰਬਚਨਕਮਾਵਣੇਸਚਾਏਹੁਵੀਚਾਰੁ॥