IIRC the whole top floor of the main Detroit public library is a huge automotive archive of donated material from GM. I went to it 20+ years ago. At the time it was only open on a VERY limited basis.
If you are going for that I would make sure well in advance that you can still go into it. If old books are your thing you will also want to hit up John King books. It is just southwest of Joe Louis arena.
Do not stay in Detroit!
Stay in Troy or Auburn hills 15 and 30 minutes north of downtown Detroit. Straight up I-75. Auburn hills will be cheaper. Depending on your length of stay and your interests I can make tons of recommendations. Food attractions ETC.
please PM me your phone # and a good time to call and I can advise you.
I lived in Oakland County my entire life till 2009 when I lost my automotive engineering job.