Lynch-Rope Law — Brett Halliday

Twister and Chuckaluck spring into action to keep an elderly widow from being run off her land

Twister Malone and Chuckaluck Thompson are making their way through a West Texas canyon when they smell death. It pollutes every inch of the winding, narrow trail, so thick and foul they fear they might choke. Finally, the two wandering cowpokes emerge onto the mesa, where they encounter the site of a massacre. A whole family of deer lies dead around a small pond, their skeletons bleached by the Texas sun. Someone has poisoned the water hole.
 
Then a rider comes around the corner, rifle in hand, and gets the drop on Twister and Chuckaluck. The Widow Kelso is a hardened old woman, and she's ready to kill. Someone has been trying to drive her off her land, and if Twister and Chuckaluck don't solve the mystery of the poisoned well fast, the deer won't be the only ones lying dead in the sun.
 
Lynch-Rope Law is the 3rd book in the Twister and Chuckaluck Mysteries, but you may enjoy reading the series in any order.