I read your book as soon as it was available, and was quite impressed. I thought you did a nice job of telling the regiment's story, especially considering the relative lack of first hand accounts from members of the unit itself. Having Marcoot's memoir was a good start I'm sure; I've been wanting to read his book for years.
I think a lot of us on this message board would like to read anything you'd want to post. Missouri Union units have been ignored for years, and it's good to see information about them. In researching a couple of Missouri units, including the 2nd which spent almost all of its service alongside the 15th, I've come across the same problem you had. Letters and other first hand accounts from these Missouri units are hard to find. I gave up on the 2nd fairly early in the process; just didn't find much out there.
Anyway, fine job on the book! Need to see more Missouri regimentals out there.....