The Michigan DNR has details on two spiders found in Michigan: the northern black widow spider, Latrodectus variolus, and the extremely rare Brown Recluse (Loxosceles reclusa). The northern black widow is further described in an entomology fact sheet from Ohio State University Extension, and there's a comprehensive collection of articles on Latrodectus at arachnology.org.
Spiders of the Douglas Lake Region is a monograph written by Olive Thomas in 1952 at the University of Michigan Bio Station near Pellston, MI. It's listed on the U of Michigan's Deep Blue service, with a note "Access restricted to on-site users at the UM Biological Station".
Michigan State University's Department of Entomology has a newsy article about Michigan spiders, and some suggestions for integrated spider management. And if you want to catch spiders, there's a nice how-to on catching spiders with a headlight.
eMedicine's article on Spider Envenomations, Widow is thorough and up to date and aimed at physicians.
A comprehensive guide to black widow spiders and other spiders of medical importance is maintained by Louis Caruana, Distinguished Professor Emeritus, Texas State University-San Marcos.
If you want lots of pictures, start at BugGuide, which uses Drupal's taxonomy system to its fullest advantage to let you browse through a nice taxonomy of bugs.