Hey everybody,
I would appreciate some advice. I have in my possession about 200 letters written by my Grandmother (mostly to her father) during her Red Cross service in England and France during World War II. She died of breast cancer before I was ever born, and I wish I could have known her, but I feel blessed to have these letters.
You could do your family a favor by first putting the letters into manuscript format, perhaps with scans or photographs of some excerpts, and doing a short-run printing for your family's use.
We're doing much the same with a recently discovered cache of family photos going back into the 19th C.
Other people have written both novels and nonfiction based on collections of letters smaller than the cache you have.