Illinois State Constitution
The records of the first constitutional convention held in Illinois, along with the text of the first constitution, are available online:
Illinois State Historical Society. The Illinois Constitutional Convention of 1818. [Springfield, IL: Illinois State Historical Society]. Illinois State Historical Society Journal, v.6, no.3, October 1894.
Legislative Reference Bureau, 1919. Constitutional Conventions in Illinois. Springfield: Schnepp & Barnes, Printers. A history of Constitutional Conventions held in Illinois written before the Constitutional Convention of 1920. It also includes a list of items for consideration during the Constitutional Convention.
The Illinois State Library has digitized all editions of the ratified and proposed state constitutions, as well as reference works about the constitutions and journals of the various constitutional conventions. They are available through the Illinois State Library's Illinois Constitution web site
Constitution of the State of Illinois, 1818, printed by order of the House of Representatives.
Text of the Constitution of the State of Illinois 1848, excerpted from: Compilation of all the general laws concerning real estate and the title thereto, in the State of Illinois 1849, p. 63-106.
Constitution of the State of Illinois  (proposed, but rejected by the voters) from Journal of the Constitutional Convention of the State of Springfield: Convened at Springfield, January 7, 1862, p.1072-1114.
Constitution of the State of Illinois Adopted in Convention, May 18th, 1870, and Ratified by the People of the State, July 2d, A.D. 1870. Chicago: The Western News Company.
The Proposed New Constitution of Illinois 1922 with Explanatory Notes and Address to the People. This constitution was also not ratified.
Constitution of the State of Illinois, Adopted at special election on December 15, 1970.