First, I must start with a critique of the assignment. I did not like the question -- "which single map in the atlas is most important for state library and information professional to understand." To me, this question was read as, "which single demographic do you, as a library professional, want to focus on." This seems to be a short-sighted view of reality. There are so many factors in play that no one map should be seen as "better" than any other. There are many useful maps that provide varying degrees of "useful" information to librarians - I hesitate to be exclusionary. "Wisconsin's Past and Present" is a very informative and useful reference tool, but it's real strength is when you look at the maps as a collection - then they tell a story.
That being said, the task at hand is to answer said question. Looking through this book, several of the thematic maps jumped out to me as a possibility, but I settled on perhaps the most standard map of all. "Population & Representation" (pgs 82-83) concerns itself with congressional districts, but gives the history of population in Wisconsin. Two maps are included, one from 1882 and one from 2002, which visually maps the movement and increase in population over time. There is also a very useful chart, "Wisconsin's Population, 1836-2000," which not only reports state population, but also charts percentage of population in urban and rural areas. Brilliant! I like this map (or series of maps and charts) because it is straightforward and there isn't much to "read into" this map. It tells us where people are, and where they're not. Combine this information of where libraries are, and where they're not, and information about the fastest growing counties/cities in Wisconsin - librarians have a very powerful tool at their disposal.