Series 1 contains diaries of Cooper's ranging in dates from 1899 to 1923. They are written in day books and mainly record where Cooper went that day. In the 1900 book, Cooper kept a tabulation of votes for the Senatorial race between the "Copper King," Marcus Daly and William Clark. In the last book, 1923, in addition to notes of what he did that day, Cooper also jotted down notes regarding his literary manuscripts found in Series 2. The Daily Journal book contains a few newspaper clippings and a draft of a manuscript found in Series 2 dealing with Red Lodge, Montana. Also included in Series 1 is the diary Mariam Skeels Cooper kept during her trip through Yellowstone National Park in 1875 as well as a reminiscence of her travel west to Montana with her family. It is one of the earliest known accounts by a white woman of the park. Also included in Series 1 is correspondence between Walter and Mariam as well as to their daughter Mariam. In addition are letters Mariam Skeels Cooper received from her family back east.

Towards the end of his life, Walter Cooper began writing a series of retrospective biographical and general Montana history essays. Cooper intended to publish the essays in a book. The literary manuscripts included in Series 2 show the many revisions and organizational changes Cooper made. What remains is a linear foot of completed and fragmented essays. Due to the fact the manuscripts would eventually be set in type, Cooper kept a detailed word count of each essay. He also hired a stenographer to type his difficult handwriting into a legible format. Topics covered in his manuscripts include the Bozeman Trail, James Bridger, life in Fort Benton, the history of Bozeman, mining expeditions, the Nez Perce, the Sioux War of 1876, the National Convention of 1892, the Yellowstone Expedition of 1874, trips to Bismarck, and the founding of Red Lodge, Montana. Also in Series 2 are typed edited versions of Cooper's original manuscripts by his daughter, Mariam Cooper Bunker entitled "The Long Trail," "A Thread in the Fabric of Empire," "The War Trail" and "Treaty with Plenty Coups." "Bluebirds Nest Where Three Forks Meet" is again a story by Mariam, but more loosely based on the original manuscripts. "The Skeels Family" in folder 14 of box 2A also appears to have been written by Mariam. Folders 23 and 40 in Box 1; folder 48 in Box 2; and folder 15 in Box 2A do not appear to have been written by Cooper or Mariam. Folder 45 of Box 1 contains research material Cooper gathered on T. H. Canfield who appears in several of the manuscripts. Cooper also gathered and received research material on Chief Plenty Coups found in folder 49, Box 1 which contains a letter Plenty Coups wrote in 1888 as well as a letter and article by James B. Hubbell giving a history of Fort Benton found in folder 22, Box 2. A typed transcript of Mary E. Cook is found in folder 50 of box 1.

Walter Cooper was involved in many business endeavors. The papers found in series 3 include correspondence and legal papers regarding his mining and lumbering businesses as well as his gun shop. Also included with his gun shop are records of pelts Cooper purchased. The bulk of the papers are land deeds. The resolution creating the city park, now Cooper Park, is also found in Series 3. Since Cooper worked closely with Bunker, other business papers relating to Cooper are also located in Series 5.

Series 4 contains personal correspondence of Eugene and Mariam Bunker. Correspondence regarding legal and business matters of both Walter Cooper and Eugene Bunker are located in Series 3, 5, and 6. Most of the letters by Mariam Cooper Bunker were written to her daughter, Mariam Virginia Bunker Barnett while she and her husband, Ed, were living overseas while in the Peace Corps. Also included are letter written by Mariam Virginia Bunker Barnett, mainly about her travels with the Peace Corps.

Papers regarding legal matters and lawsuits Bunker filed for clients are represented in Series 5. The bulk of the material contains correspondence and legal papers regarding the Lee Gold Mining Company's legal actions from 1923 to 1926. Also contained in the papers is correspondence in relation to the closing of family estates, including his own, his mother Elizabeth, and his brother George.

Series 6 contains correspondence and legal papers for various businesses Bunker represented. The majority of the information includes papers for Bunker Brothers, an automobile dealership two of Eugene's brothers owned and operated in Illinois. Other material includes mining companies Cooper ran such as the Pass Creek Mining and Smelting Company and the Havana Mining Company. Although Series 6 contains the bulk of the information on these companies, additional information can also be found in Series 3.

Mariam Cooper Bunker kept memorabilia of family records such as the school records and family papers of her children, family records including Daughters of the American Revolution material, and an alumni bulletin from National Park College (Seminary). Also included in Series 7 is the 80th Anniversary Edition published in 1937 which contains information of the Bunker family. Also are newspaper clippings and memorabilia from the Barnett family.

Series 8 contains photographs and a photo album. The album contains pictures when Mariam Cooper Bunker attended National Park Seminary, graduating in 1909. Pictures of the buildings of the school include the Odeon Theater, the Villa, Theta Mission, the Windmill, Chiopi Bungalow, the Gymnasium, the Pagoda, the Main House, Senior House, Pi Beta Nu Castle, Zeta Chalet, and the Chapel. Other pictures include students at National Park Seminary, a track meet held at the school, and pictures of Washington, D.C. Other photographs in Series 8 include pictures of Walter Cooper, Mariam Virginia and Elizabeth Bunker, Eugene and Mariam Bunker, Eugene Bunker, Jr., and Ed Barnett, Jr.