If you are researching Michigan ancestors, you may be interested in this site, Michigan Biographical Index. "This is a cumulative full-name index merging many Michigan publications and manuscripts," says the introduction. You search for an ancestor's name, and if found, you will be given information about the publication his (or her) biography is printed in. Some of these sources may be available online, for instance, at the Michigan County Histories site or on Michigan GenWeb or GenNet sites. You can Google the name of the publication; in fact, it may even be available at Google Book Search! Don't forget your local genealogical or historical society's library, or try InterLibrary Loan at your local public library to obtain a desired book. Many of these can also be purchased new or used at online bookstores.
Now go out and find your ancestors!