One part of any lesson that Andrew would write about his life in Blaenelly would have to embrace the idealism of the medical profession. It is in Blaenelly that he enters the medical profession. The letter would have to contain this zeal and enthusiasm that Andrew has towards his profession. I think that it should communicate the perspective of one who is fresh out of school and is eager to put their theoretical learnings into action. Blaenelly is the perfect location for Andrew because it demonstrates his ability to "do something" in a setting that demands it. This spirit should be evident throughout the letter.
Another part of the letter that should emerge is how Andrew's idealism and hope leads him to Christine. It makes sense that she would be a school teacher, another profession where hope and idealism furthers one's initial entry. Writing from the point of view of his time in Blaenelly, Andrew's letter should encompass his sense of hope and optimism in both the professional path he has chosen and the town where he plans to carry this out. This might be able to extend into Andrew's association with Dr. Denny. For Andrew, life in Blaenelly is defined by all that awaits and all the hope he carries. The letter should indicate that.
Finally, it might make sense to outline some of the challenges that Andrew faces. These issues end up testing his idealistic commitment. One challenge that can be communicated is that the townspeople of Blaenelly do not possess much in way of modern thinking about medicine, something that causes Andrew to be viewed in a skeptical light. The letter that Andrew writes to his friend can reflect this. Another condition of life in Blaenelly that Andrew might hint at in the letter would be how Dr. Page is not as diligent and focused as how Andrew might expect a practicing physician to be. This is another aspect of life in Blaenelly that Andrew could illuminate in his letter.