The majority of this series is personal correspondence sent to Harrison, although there are also a significant number of items that were sent to Harrison in his official capacity as Mayor of Chicago or Collector of Internal Revenue. Several letters have handwritten annotations by Harrison explaining the letter's context or giving his thoughts on the sender or the letter's subject.

Much of Harrison's official incoming correspondence involves patronage job appointments. The rest of Harrison's incoming correspondence covers a wide range of topics, including: (a) his three books (Stormy Years, Growing Up With Chicago, and With the American Red Cross in France, 1918-1919); (b) the political activities of the Democratic Party at both the local and national level, including four letters from Tammany Hall boss Richard Croker; (c) early Chicago history; (d) hunting and fishing trips; (e) efforts to locate the whereabouts of various individuals with whom Harrison was acquainted in the past; and (f) responses from well-known people of Harrison's day from whom he requested autographs as a young man.

Among the correspondence in this series are two interesting letters from then Senator Harry Truman in 1936 in which Truman tells Harrison what he thinks of the French and expresses his displeasure at France's failure to repay the United States for debts incurred during World War I in connection with the purchase of war supplies. There is also a letter from Harrison's brother, William Preston Harrison, giving his eyewitness account of the assassination of Harrison's father in 1893, and a letter from Lawton Parker inviting Harrison to attend a meeting to discuss the formation the Arts Club of Chicago. Finally, this series includes letters relating to Harrison's service with the American Red Cross in France at the end of World War I, and his gifts to the Art Institute of Chicago.

There is a fair amount of correspondence (i.e., over five letters) from the following individuals or entities: American Red Cross; Art Institute of Chicago; Bobbs-Merrill Company; William Jennings Bryan; Charles Collins; Charles G. Dawes; Charles S. Deneen; Edward F. Dunne; E. K. Eckert; James Farley; Alexander Hugh Ferguson; Charles Fitzmorris; Sophonisba Preston Harrison; William Preston Harrison; Henry Horner; Cordell Hull; Harold L. Ickes; James Hamilton Lewis; Frank O. Lowden; Edgar Lee Masters; William Gibbs McAdoo; John T. McCutcheon; F. Millet; Henry Morgenthau Jr.; Battling Nelson; Lawton Parker; Henry T. Rainey; Frederick Rex; Franklin Delano Roosevelt; Julius Rosenwald; A. J. Sabath; Adlai E. Stevenson; William Hale Thompson; Henry Emerson Tuttle; and Walter Ufer.

Letters to Harrison specifically about his family's genealogy and history are arranged separately in Series 11 (Harrison Family History). Letters to Harrison about the Chicago Commission for the Encouragement of Local Art are arranged separately in Series 12 (Chicago Commission for the Encouragement of Local Art).