I've been reading a couple of books on the Fayette County/Round Top area. The first I just finished was on Nassau Plantation, Nassau Plantation: The Evolution of a Texas German Slave Plantation. I found the book fascinating. The stories about families I knew about and/or are connected with Round Top is great background information for the Henkel Square events besides just being great stories on the immigration of Germans to the area.
The other book I've had for awhile, Ancestral Voices. This is transcriptions of several letters from the Von Rosenberg family from the Round Top/Fayette County area. I had read some parts, mostly those dealing with the 1860s but have now started from the beginning.
Both books offer great insights into the lives of the German immigrants to Texas.