I think that any letter from Andrew about his life and work in Blaenelly would have to speak of his experiences in the town. At the outset of the novel, Andrew is excited and filled with enthusiasm about his chosen profession of medicine and the role he is to play in the lives of the Blaenelly population. It would make sense that he would describe his initial impressions of the town upon seeing it for the first time and the combination with his emotional state, as a result:
The tops of the mountains were hidden by a grey sky; and their sides, which contained many coal mines, looked black and cold. Night was falling and no trees, no grass could be seen. At a bend in the railway line, a bright red light from an iron works suddenly came into view, lighting up a number of men who were working with all their strength. At once, a sense of power filled the mountain valley. Manson drew a deep breath. He felt an added call to effort, a sudden hope and promise for the future.
Andrew's letter to a friend would underscore how potentially overwhelmed he is, as evidenced in the naturalist description. Yet, the "call to effort" and "sudden hope and promise for the future" are the animating spirits within Andrew and his views on life and work in Blaenelly. There is much that would overwhelm others. However, Andrew is so zealous and enthusiastic about what he is going to do and what he is going to undertake that it is almost a compensation for it.
Another part of his letter could describe the first house call he made. The palpable feelings of excitement, fear, and intensity are all evident in this moment, enough to relive the experience in a letter: "As Andrew went- ever to her bed, he suddenly felt a great sense of responsibility. He was alone. He must find out what was wrong with the woman and cure her - with no other doctor to advise him." A letter would emphasize the "great sense of responsibility" as well as his fundamental desire "to cure" those who are sick. Andrew sees his life and work in Blaenelly in these distinct terms and he would communicate this to an outsider in emotional language that is reflective of the world and his place in it.
Finally, I think that Andrew would be honest to suggest in the letter that his role is a vital one. It becomes clear that there are not many other alternatives available to the people in Blaenelly. Denny talks to Andrew about the state of affairs in the most direct terms when he says that the existing medical system in the town is "very bad" in that "there is no hospital, nor anything else that a doctor needs." Andrew would communicate to his friend what Denny indicates about the mortality in the town, a situation where "people often die of fever" and ingest bad drinking water as its cause. Denny makes it clear that Andrew is really the only hope in the town because Page is sick and dying, Nicholls "thinks only about making money," Bramwell "knows nothing," and Denny, himself, shows a penchant for alcohol. Andrew would make it clear that his life and work in Blaenelly are motivated through the important and essential role he plays in providing quality medical care to people in need. These views are so integral to Andrew's sense of professional and personal being that he would illuminate them in a letter.