Since I belong to the Garage Sale School of Genealogy, having discovered more things by accident than I ever did on purpose, I really like manuscripts. Manuscript collections are normally indexed with a very sketchy description, and you have to dive into them to see what might pop to the surface.
The Indiana State Library has a great manuscript collection and I have spent many productive hours there and have located numerous data sets for local and state publications.
I suggest you spend some time looking into their card catalog (remember those things that do not come with a computer screen?). They have wide variety of topics and you might be surprised at what you find.
You order your files from this reading area and the staff will bring them to you one by one.
Brent Abercrombie is normally the librarian on duty here.
The regular card file gives you the info you need to order manuscripts. The section at the far right contains a subset of special collections.
The other side of the card case has a separate set of cards for photographs and images.
Explore manuscripts here and at other places--they are an interesting and rewarding resource.