SOURCES OF BACKGROUND INFORMATION IN ERB'S APACHE NOVELS
Burroughs drew upon his personal experiences from when he served as a trooper
with the U.S. 7th Cavalry at Fort Grant, Arizona.

He also cited the following books and periodicals 
as being among the reference materials he used:
Part I in ERBzine 1153

The Marvellous Country: Three Years in Arizona and New Mexico, the Apache's Home by Samuel Woodworth Cozzens 1873
Thrilling Days in Army Life by General. George. A. Forthsyth 1900
Lives of Famous Indian Chiefs by Norman B. Wood
The Frontier Trail by Colonel Homer W. Wheeler
The Land of Poco Tiempo by Charles. F. Lummis

Part II in ERBzine 1154 (below)

Geronimo's Story of His Life by S.M. Barrett, 1907
Trailing Geronimo: The Outbreak of the White Mountain Apaches, 1881 - 1886  by Anton Mazzanovich
Life Among the Apaches by John Carey Cremony
Apache Medicine-Men by John G. Bourke

OTHER
More ERB Apache References in ERBzine

Geronimo, a Bedonkohe Apache leader of the Chiricahua Apache, led his people's defense of their homeland against the U.S. military after the death of Cochise. In the early 1870s, Lieutenant Colonel George F. Crook, commander of the Department of Arizona, had succeeded in establishing relative peace in the territory. The management of his successors, however, was disastrous. In 1874, some 4,000 Apaches were forcibly moved by U.S. authorities to a reservation at San Carlos, a barren wasteland in east-central Arizona. Deprived of traditional tribal rights, short on rations and homesick, they revolted. Spurred by Geronimo, hundreds of Apaches left the reservation to resume their war against the whites. In 1882, Crook was recalled to Arizona to conduct a campaign against the Apaches. Geronimo surrendered in January 1884, but took flight from the San Carlos reservation in May 1885, accompanied by 35 men, 8 boys and 101 women. Crook, along with scouts Al Sieber, Tom Horn and Mickey Free (the white child Cochise was falsely accused of abducting) set out in pursuit, and 10 months later, on March 27, 1886, Geronimo surrendered at Cañon de Los Embudos in Sonora, Mexico. Near the border, however, fearing that they would be murdered once they crossed into U.S. territory, Geronimo and a small band bolted. As a result, Brigadier General Nelson A. Miles replaced Crook as commander on April 2. During this final campaign, at least 5,000 white soldiers and 500 Indian auxiliaries were employed at various times in the capture of Geronimo's small band. Five months and 1,645 miles later, Geronimo was tracked to his camp in Mexico's Sonora mountains. At a conference on Sept. 3, 1886, at Skeleton Canyon in Arizona, Miles induced Geronimo to surrender once again, promising him that, after an indefinite exile in Florida, he and his followers would be permitted to return to Arizona. The promise was never kept. Geronimo and his fellow prisoners were put to hard labor, and it was May 1887 before he saw his family. Moved to Fort Sill in the Oklahoma Territory in 1894, he at first attempted to "take the white man's road." He farmed and joined the Dutch Reformed Church, which expelled him because of his inability to resist gambling. He never saw Arizona again, but by special permission of the War Department, he was allowed to sell photographs of himself and his handiwork at expositions. Before he died at Fort Sill, Oklahoma, Feb. 17, 1909, he dictated to S.S. Barrett his autobiography, "Geronimo: His Own Story." 
http://www.desertusa.com/magfeb98/feb_pap/du_apache.html
Geronimo:(Goyathlay) (c. 1829-1909) Encyclopedia of North American Indians

The Snake Dance of the Moquis of Arizona: Being a Narrative of a Journey from Santa Fe, New Mexico, to the Villages of the Moqui Indians of Arizona ~ reprinted by Kessinger Publishing (August 2003)
1884. The author has endeavored to present a truthful description of religious rites of the Moqui, the very existence of which is known to but a few of our people, and a narrative of incidents which may serve to entertain and amuse, if they do not instruct, those into whose hands this book may fall. The Moqui Indians inhabit seven different villages, situated a few leagues apart. In treating them all, it was impossible to avoid a seeming repetition of description, where the same apparel, food, arms and ceremonies were to be explained. This work is a narrative of a journey from Santa Fe, New Mexico, to the villages of the Moqui Indians of Arizona. Illustrated.

The Diaries of John Gregory Bourke: November 20, 1872, to July 28, 1876 ~ reprinted by University of North Texas Press (March 2003)
 

Danton Burroughs found the following note with the manuscript for The War Chief:

"I have gone over the 'copy' carefully and have indicated a number of phrases, sentences and paragraphs deleted by them, which I wish to have retained.

The preparation of the manuscript required considerable research work and as it is necessary for the reader to be able to understand the viewpoint of the Indian, if he is to be in sympathy with the principal character, it is essential that much of the matter deleted should remain even though it draws comparisons that may be odious to some people of our own race and sometimes shocking to people whose religious convictions are particularly strong. 

I should also call your attention to an Indian name and an Indian word concerning which the magazine editor and I seem not to agree. 

The name is that of a famous Apache Chief, Mangas Colorado, variously spelled Mangus and Magnus. From a very old book I obtained the suggestion of the derivation of this name, which in Spanish means colored sleeves. The author supposed that the name may have been given to him by the Mexicans, either because of the garment he wore with colored sleeves or from the fact that his sleeves or arms were stained with the blood of his victims. 

The other word to which I refer is Izzo-Kolth, which the magazine editor insisted on changing to Izze-Colth. My authority for this spelling is an article by John G. Bourke, THE MEDICINE MAN OF THE APACHES, which appeared in the annual report of the Bureau of Ethnology for 1887 and 1888. 

The magazine editor deleted what evidently appeared to him tiresome descriptions of Indian customs, such as burial ceremonies and the decoration of the bodies of medicine men, but as there is not a great of this and I believe that it is all based on good authority, it should be permitted to remain."