I've been working from the informatoin on this obituary for an upcoming issue of Casefile Clues.
However sometimes there is a point where online work needs to stop and work in offline sources is necessary. This obituary comes from the Quincy, Illinois Daily Journal of 17 September 1913.
Based upon information in the obituary, I have located Agnes Harper in the 1880-1910 censuses that are extant in West Point, Illinois.
However, I have not had much luck locating her in census record before 1880 and I think that it's time to do some onsite research in the county where she died in 1913 and where she apparently lived from 1880 until her death.
Cemetery records or transcriptions might provide the year of death for her husband. There may potentially be probate information for him as well. The local historical society may have additional information on the Harper family that I can't access online.
Once I've searched probate records (both for Agnes Harper and her husband) and local historical society records, I will be better equipped to search for Agnes in other records.
Sometimes one has to stop and regroup before continuing online research when offline sources may answer some questions.