Thanks to the St. Clair County (IL) Genealogical Society Quarterly for this alert:
Approximately 30 volunes of Transactions of the Illinois State Historical Society are available online at The Internet Archive (www.archive.org). Among the published articles are transcriptions of pioneer letters [and] historical essays on a variety of subjects.I own a few of these volumes -- collections of papers from ISHS annual meetings early in the 20th century -- and they can have cool old stuff. Trouble is they weren't well indexed before.
The new (volume 32, number 2) issue of this quarterly includes 1888 death register extracts, 1825 plats of Illinois City and Belleville, 1858 Belleville tax list, and part of a 1913 marriage index.