Here's a good link for finding in which county a particular city or town is located. http://www.hometownlocator.com/
Yes - Wayne County is what you need for Detroit area records. http://usgwarchives.net/mi/wayne.htm
I think it's premature for you to look at the 1940 Census, as Detroit is very large, and where would you start looking? I was hoping to find him in a U.S. City Directory for about that year, but no luck there either. That would tell you where he was in the 1940 census.
A good site for help with the 1940 Census is SteveMorse.org.
I'm getting obsessed too! haha
p.s. Don't get fired! ;-)