Angela:
I hate to dodge your question but the answer is that it really depends on the substance of the letters. I just published the diary of a Union marine and the diary (or Note-Book as he called it) was strong enough that it was easy enough to make into a book with a lot of annotations and some preliminary material putting it in context. There are some books like that with letters but it is harder to do that without lengthy explanatory introductions to each of the letters setting the context. Just based on what you said, I would suspect that you might want to do chapters that divide his service up by battles or campaigns and then quote the letters in small parts maybe with the full text in an appendix.