Illinois Harvest is digitizing books from the University of Illinois Urbana-Champaign library faster than I can keep up with (check the tag cloud for previous posts). Recent additions of potential genealogical and microhistorical interest:
Portrait and Biographical Record of Macon County, Illinois. Chicago: Lake City Publishing Co., 1893. 736 pages on the city of Decatur and its immediate hinterland.
The Indian tribes of the Chicago region, with special reference to the Illinois and the Potawatomi, by William Duncan Strong. Chicago: Field Museum of Natural History, 1926.
A Visit to the Illinois Eastern Hospital. Chicago: H.O. Shepard, 190_. That's the mental institution better known as "Manteno," where a grandaunt of mine was later an inmate.
Army Life of an Illinois Soldier, including a Day by Day record of Sherman's March to the Sea, by Charles Wright Wills. Wills had a busy war, serving in the 8th Illinois Infantry, 7th Illinois Cavalry, and 103rd Illinois Infantry. Use this link to the American Libraries site; the others bring up a 404 Not Found error on my machine.
Chicago Daily News National Almanac for the years 1892-3, 1896, 1898-1905, 1909-9, 1911-17, and 1919-23.
Illinois State Gazetteer and Business Directory for the Years 1864-5, Embracing Descriptive Sketches of All the Cities, Towns and Villages Throughout the State... [well, you get the idea]. Chicago: J.C.W. Bailey, 1864. 846 pages. Downstate coverage is good, including even a brief mention of the still-unincorporated hamlet of Summum in SW Fulton County.
Two of Edwards' annual Chicago city directories, volume 12 for 1869-1870, and volume 14 for 1871.
Industrial Chicago, volumes 1-5. Chicago: Goodspeed, 1891-1894. Volumes cover "the building interests" (v1&2), "the manufacturing interests" (v3), "the commercial interests" (v4), and "the lumber interests" (v5).
Album of Genealogy and Biography, Cook County Illinois, for 1897, 1899, and 1900, although I'm not sure the later volumes add much to the first one. As in all such high Victorian productions, expect to find only the rich and well-known telling their own highly selective versions of the story. If you need George Pullman's take on the Pullman Strike, you can find it here in all its rigid, archaic glory.
The book of Chicagoans: A Biographical Dictionary of Leading Living Men and Women of the City of Chicago. Chicago: A.N. Marquis, 1905 and 1917.
Remember, these are digital images of the originals, totally searchable -- the gold standard AFAIK.