Following is the information available (from the State of Illinois
Archives website): REGISTERS OF PRISONERS. August 29, 1833-November 31,1841; May 15, 1847-June 27, 1975. 172 vols. Index (RS 243.201).

Registers are of prisoners received at the Alton prison (1833-1860) and the Joliet/Stateville site (1860-1975). Registers variously provide the prisoner's registration number and name; date received; names of aliases used; name of the county in which the crime was committed; description of the crime; sentence term; prisoner's plea; his age, height,complexion, hair and eye colors, birthplace, and length of residence in Illinois; indication that his parents are living or dead; their residence; his marital status and, if married, his wife's residence; his religious affiliation, habits (e.g., drinks, smokes, chews), and education; the term of the court which convicted him; and his disposition (e.g., discharged, paroled, escaped, sent to insane asylum,died).

Beginning October 1885 also provided are the prisoner's weight,
occupation, former prison record, age he left home, parents'
birthplaces, the number of children he fathered, and his prison work
assignments, conduct record, and medical treatment history. After July
1895 the prisoner's race, names of associates, hereditary diseases,
conditions of heart and lungs, addresses of correspondents, and parole
record are given. Starting January 1914 the prisoner's naturalization
status is provided and after July 1936 indication of past service in the U.S. armed forces is given.

For the period February 15, 1854-September 27, 1878; September 14,
1881-September 3, 1885 a separate commutation section is provided. It
shows the date of commutation, the prisoner's registration number and
name, the name of the county with jurisdiction, the length of the
original sentence, the length of the commuted sentence, and the name of the Governor granting the commutation.

For those who might have had an ancestor or relative incarcerated there, this appears to be a gold mine of information. Here's the link to the Illinois Trails History and Genealogy page for more information:
http://www.iltrails.org/convictregister.htm

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