Intro
It seems that after MGA died, Nooruddin began collecting all the letters that MGA and his team wrote and published them into book form, not all of them were published however, nor is it clear as to how many copies were made and if the originals still exist, we are sure of Ahmadiyya editing, so we are careful.  As we all know, Pir Siraj ul Haq Nomani wrote most of MGA’s letters, and on MGA’s behalf, MGA most likely dictated what needed to be written.

The reference is from Hidden Treasures, which is a book that summarizes the writings of MGA and his team
https://www.alislam.org/library/book/hidden-treasures-of-islam/

See the introduction section

Maktubat-e-Ahmadiyyah

“””Letters written by the Promised Messiahas from 1883 to 1908 to various personalities and some of these letters relate to the importance of the Khilafat and about the Companions of the Holy Prophetsa are collected in these volumes.

There are seven volumes of letters. The fifth volume comprises five parts. Details of these volumes are given as:

Volume 1
One hundred and sixteen pages, published in 1908, in Urdu. This comprises a collection of letters that the Promised Messiah had written to Mir ‘Abbas ‘Ali Shah Ludhianvi from October 1882 to June 1885.

Volume 2
Ninety pages, published in Lahore in 1912, in Urdu. This comprises a collection of letters written by the Promised Messiah from 1878 to 1903 to various Hindus and the leaders of the Aryah Samaj, including Indarman Muradabadi, Pandit Dyanand, Pandit Lekh Ram, Bawa Shiv Nara’in and Ram Charan, in which the Promised Messiahas explained the truth of Islam and fixed a period of one year for anyone of them to come and stay with him in order to witness a Divine sign and he also drew a comparison of the teachings of the Holy Quran with the Vedas.

Volume 3
One hundred and twenty-two pages, published in Lahore in 1914, in Urdu, is a collection of letters written by the Promised Messiahas from 1884 to 1903 to Christian missionaries, including Alexander Dowie.  He asked the Christian missionaries to produce a comparable
observation about Jesus as the one made about the Holy Prophetsa by the King of Ethiopia in his time. He offered them a reward of Rs. 1,000 for it. The Promised Messiahas effectively answered all objections raised by Christians about the Holy Prophetsa and established the truth of the Holy Prophetsa with cogent arguments. An Introduction to the hidden Treasures of Islam

Volume 4
Forty pages, published in Qadian, in Urdu, comprises a collection of the letters written by the Promised Messiahas to Maulavi Muhammad Hussain Batalvi, in which he marshalled arguments about his truth and adduced and enumerated signs which had taken place in his favour to establish his truth.

Volume 5: Part I
Fifty-six pages, published in 1918, in Urdu, is a collection of 96 letters written by the Promised Messiahas from 1892 to 1905 to Seth ‘Abdur Rehman of Madras.

Volume 5: Part II
One hundred and twenty-eight pages, published in Urdu, is a collection of 88 letters written by the Promised Messiahas from March 8, 1885 to August 26, 1892 to Hadrat Maulavi Hakim Nur-ud-Din. These letters are in his own hand and relate to the efficacy of prayers and specially those made at the eleventh hour. Mention has also been made of some of his revelations specially about the Promised Reformer and also contain suggestions about the printing of the magazines.

Volume 5: Part III
One hundred and seventy-six pages, published in 1929, in Amritsar, in Urdu, is a collection of 278 letters written from 1884 to 1902 to Ch. Rustam ‘Ali, in which he praised the financial assistance rendered by him and other matters.

Volume 5: Part IV
One hundred and forty-four pages, published in 1935, in Amritsar, in Urdu, is a collection of 62 letters written from 1890 to 1906 to Nawab Muhammad ‘Ali Khan in which a number of different subjects have been discussed, including the importance of bai‘at, signs of the Promised Messiah, the method of Istikharah, unreasonable wavering of the people on the prophecy of ‘Abdullah Atham and the true significance of Khatm-e-Nubuwwat.

Volume 5: Part V
Thirty-two pages, published on June 10, 1944, in Hyderabad Deccan, in Urdu, is a collection of 270 letters addressed to Munshi Ahmad Jan Ludhianvi, and other friends in Kapurthala, Hadrat Nawab Muhammad ‘Ali Khan of Jhajjar, Munshi Habibur Rehman, Munshi Dafar Ahmad, Khan Sahib ‘Abdul Majid and Maulavi ‘Abdullah Sanauri, and some other letters written in his own hand.

Volume 6
It is a collection of 23 letters written by the Promised Messiahas to Hadrat Maulavi Hakim Nur-ud-Din and a number of different non Ahmdi scholars. This collection was edited by Sheikh Ya‘qub ‘Ali Khan ‘Irfani.

Volume 7
This collection was compiled in 1954 by Malik Salah-ud-Din, M.A. This has 51 letters, 36 of them were addressed to Nawab Muhammad ‘Ali Khan of Maler Kotla between 1892 to April 1908, and some others addressed to a number of different personalities including Bhai ‘Abdur Rahman Sahib Qadiani and Muhammad Ibrahim Khan (Sindh).

Note: Nazarat Isha‘at has combined all the above letters (and some more letters which could not be included in the seven volumes) into three volumes and published them in 2008.