In addition to the website mentioned in another reply the other resources I have found during extensive research in Allegan County are:
The archieves at Western Michigan University, Kalamazoo, try to go when school is not in session - very limited parking. They have thousands of original documents which have been donated by families in addition to the usual genealogy resources.
I recently got a copy of an original 1906 birth certificate from the city clerk in Otsego. They have a limited number of original certificates if the child was born in that city.No fee!!!
The Kalamazoo Public Library. lots of old city directories and newspaper archieves on microfilm.
The Michigan Collection at the State Library of Michigan, Lansing.They have nearly every one of the volumes that were published by Ruth Monteith, a local researcher, detailing births, deaths, marriages, etc..Rated one of the ten best genealogy libraries in the US. http://www.libofmich.lib.mi.us/http://www.libofmich.lib.mi.us/
There is an Historical Society housed in the old jail in downtown Allegan.They have some old genealogy documents and its worth a look.Limited hours.
Finally, write to: Mrs. Donna Benedict, 129 Cedar St., Wayland, Mi. 49348. She is a researcher for the Wayland Area Tree Tracers Genealogy Society. They have a tremendous amount of obiturary information as well as general genealogy information about the local area of Wayland and Allegan County and are very, very helpfull.