Another book, “The Duke of Duval -The Life & Times of George B. Parr” offers an excellent background but that book is extremely hard to find. Amazon has it priced at $238.
I used to work in that county back in the ‘90s. It’s about 98% Hispanic and a poor county. The Parr family still had famiily with land holdings just outside Benavides.
Another good one, if you can find it, is “A Texan Looks at Lyndon,” by J. Everts Haley. Out of print, but still seen now and then in used-book stores.