Looking at the roster of Co.K, 17th Texas Cavalry (dismounted) in "This Band of Brothers" book by James McCaffrey, the writer of the letter was in what would eventually be called Granbury's Texas Brigade.
Obviously he (by date of letter)was captured at Akansas Post, and one of many sent up to Camp Douglas, and other prison camps.
They saw too many mass graves, (all over the country) back then.
Thanks for posting the letter.
Kevin Dally