These are links to repositories that have been essential to my research, as well as a list of recommended reading if you should wish to concentrate on a particular topic. Hopefully, this material will be beneficial to your own study.

Repositories
The Texas Ranger Hall of Fame and Museum, Waco, Texas
Texas Ranger Heritage Center, Fredericksburg, Texas
Texas State Library and Archives Commission, Austin, Texas
Dolph Briscoe Center for American History, Austin, Texas
Historical Research Center, Texas Heritage Museum, Hill College, Hillsboro, Texas
The Texas Collection, Baylor University, Waco, Texas
Haley Memorial Library and History Center, Midland, Texas
Kansas Historical Society, Topeka, Kansas
U.S. Cavalry Museum, Fort Riley, Kansas
Midwest Genealogy Center, Independence, Missouri
Western History Collections, University of Oklahoma Libraries, Norman, Oklahoma
Recommended Reading
Texas Rangers: Lives, Legend, and Legacy
by Bob Alexander and Donaly E. Brice
Texas Ranger: The Epic Life of Frank Hamer, the Man Who Killed Bonnie and Clyde
by John Boessenecker
The Texas Rangers: Wearing the Cinco Peso, 1821-1900
by Mike Cox
Time of the Rangers: Texas Rangers: From 1900 to the Present
by Mike Cox
Firearms of the Texas Rangers: From the Frontier Era to the Modern Age
by Doug Dukes
Tracking the Texas Rangers: The Nineteenth Century
edited by Bruce A. Glasrud and Harold J. Weiss, Jr.
900 Miles on the Butterfield Trail
by A. C. Greene
by Pekka Hämäläinen
The Seventh Star of the Confederacy: Texas during the Civil War
edited by Kenneth W. Howell
Riding for the Lone Star: Frontier Cavalry and the Texas Way of War, 1822-1865
by Nathan A. Jennings
The Mason County “Hoo Do” War, 1874-1902
by David Johnson
Texas Ranger Captain William L. Wright
by Richard B. McCaslin
by Rick Miller
Savage Frontier: Rangers, Riflemen, and Indian Wars in Texas, Volume IV, 1842-1845
by Stephen L. Moore
The Sutton-Taylor Feud: The Deadliest Blood Feud in Texas
by Chuck Parsons
A Lawless Breed: John Wesley Hardin, Texas Reconstruction, and Violence in the Wild West
by Chuck Parsons and Norman Wayne Brown
Frontiersmen in Blue: The United States Army and the Indian, 1848-1865
by Robert M. Utley
Frontier Regulars: The United States Army and the Indian, 1866-1891
by Robert M. Utley
Lone Star Justice: The First Century of the Texas Rangers
by Robert M. Utley
Lone Star Lawmen: The Second Century of the Texas Rangers
by Robert M. Utley
The Comanches: Lords of the South Plains
by Ernest Wallace and E. Adamson Hoebel