Proposed Biography of Elwin Linman Rockwell

My paternal great-grandfather Elwin Linman Rockwell was my dad's hero. Dad told us kids stories about his grandfather (the only grandparent he knew). Because he lived close to him, he visited his grandfather often. And he had some great memories of the man.
When I was 24 years old (a long time ago) I became interested in genealogy and family history and for the next 30 years I questioned my dad continuously about family matters. It only stopped when my dad died. I especially wanted to know more about Elwin. One time when I used his grandfather's given name, trying to keep the grandparents straight, my dad cringed and said, "Granddad must be rolling in his grave to hear you say that."
When I took a writing class in the early 80's I wrote a character sketch of Elwin, from dad's information. The teacher said it would make a great article if it was expanded, but I had no idea how to research it. Fast forward twenty-some years with more writing and genealogical searching experience, after reading a book on how to write a family history, I decided I was ready to write a Biographical narrative on Elwin's long and varied life.
I knew that first I needed to do more research, even though I had quite a lot, there were many gaps. I've been working on this about a year and I'm still not finished. I am on the lookout for any info on this man. I have searched in the courthouse, death, marriage and land records. I need to get any probate records there are. Also online I have searched census and other records. And I've spent a good deal of time searching the local newspapers.
I have photos and some family papers, such as a marriage certificate, but I would dearly love some letters and/ or correspondence or journals/diaries. If anyone has any of these items or knows where some of these are, I'd be interested. I'd be willing to share what I have collected which is considerable.