My experiences with donating items to museums is hit-or-miss. I've had better luck with smaller historical societies than I have with larger state-run ones. A long-term loan is also something to keep in mind. That way you don't have to worry about the museum selling it off.
In terms of a book, unless you'd be doing it as a labor of love, I'm not sure there will be enough interest to justify the cost of such an endeavor. The 104th OVI is probably one of the most well-documented regiments there is. In addition to two regimental histories, there are at least four published letters/diaries in book form, four more unpublished diary transcripts that I know of, a masters thesis, and a magazine article about Harvey. That doesn't include the National Tribune articles, the articles in the Ohio Genealogical Society newsletters, or CPT David Bard's diary (Company I) in the Western Reserve Historical Society. But, if you're truly interested in doing a book, it might be best to just self-publish the letters by themselves with maybe a little bit of amplifying text. Or send the transcript to one of the various CW magazines/journals to see if they'll publish them.
BTW, Harvey's collar came up for sale about ten years ago. I don't know who got it, though.
I hope this helps.
Jim