"My great grandfather Henry John Rademacher was born in Milwaukee in 1857 and died there in 1920. ... I'm looking for information on Henry John beyond what I can find in federal census records."
A Henry J. Rademacher died in Milwaukee in April 1920 and his death notices and an obit. of some sort (honoring the deceased, but not necessarily his descendants) are found in Milwaukee's German language newspapers. These newspapers, and an index under construction, remain in local libraries. There are about 60 Rademacher entries or references in the current data and some other surnames connected to Rademacher include Wagner Bohnsack Kropp.
"Milwaukee's German Newspapers; an index of death notices and related items" (1844-1950) / an ongoing, independent project created, compiled and edited by Gary R. Rebholz.