A year and a half ago, I wrote a post titled, "County Histories and Biographies (Mug Books)," in which I listed numerous online digital book sites where you can find these rich resources. Since that time, I have found more state-specific sites, either at the state websites themselves, or on genealogical web-hosting sites. Some have text-only pages and/or only have a few county books listed, while others have dozens of mug books in glorious digitized format. I hope these continue to be helpful in your research.
California County Histories and Biographies links - http://www.cagenweb.com/cpl/cpl_bio.htm
Idaho History and Biography links - http://www.idahogenealogy.com/index.htm
Illinois Harvest - http://illinoisharvest.grainger.uiuc.edu/index.asp - do a search for "county history" or "county biography"
Indiana County History - http://www.countyhistory.com/ - they also have biographies
Iowa County Histories - Digital Collections of the Iowa State University - http://cdm16001.contentdm.oclc.org/cdm/search/collection/p15031coll6
Kansas History and Heritage Project - http://www.rootsweb.ancestry.com/~kahhp/onlinecounties.html
Michigan County Histories and Atlases - http://quod.lib.umich.edu/m/micounty/
Missouri County History Project - https://www.sos.mo.gov/archives/mdh_splash/default.asp?coll=mocohist
Montana Memory Project - http://cdm103401.cdmhost.com/cdm/landingpage/collection/p15018coll43
New York State History - Towns and Counties - http://www.nysl.nysed.gov/scandocs/towns.htm
North Carolina - NCpedia.org - check out the "Biography & People" tab
Pennsylvania County Histories and Biographies links - http://www.usgwarchives.net/pa/pahist.htm
Tennessee - from Goodspeed's History of Tennessee - http://www.tngenweb.org/records/goodspeed.html
Washington - http://www.historylink.org
Wisconsin Local and County Histories - http://preview.wisconsinhistory.org/Content.aspx?dsNav=N:1175