The best books I have read are:

Between the Enemy and Texas: Parsons' Texas Cavalry in the Civil War, by Anne Bailey

The Ragged Rebel: A Common Soldier in W. H. Parsons' Texas Cavalry, 1861-1865, by B. P. Gallaway

Hill College in Hillsboro, Texas also has a lot of documents and history on the Texas 12th (type in "Hill College, Texas" on your search engine)

There are a lot of web sites on the 12th. Again, type "12th Texas Cavalry" in you search engine.

Below is what I found on your ggfather on the web at one of the sites. Yellow Bayou was the last battle fought in the Red River Campaign and the last fought by the 12th.

Hanson, John, pvt,pvt; 1Sep63-1Jan64, "Wounded in Battle, May 18th 1864. Absent."; J&F65, enl 12 May 64, Yellow Bayou, LA, "Absent from wound received in battle, May the 18th, 1864, on Red R.R."