the odd women
I second George Gissing’s The Odd Women, but it’s remarkable how few examples there are:
It is interesting that the middle-aged single heroine shows up as detective in mysteries (male detectives are also often single). The isolation of the detective is part of the territory. In serious fiction, there is a real dearth. Most fiction is about young people, and love/marriage is a big big subject for novels. the pressure toward a wedding (or a divorce!) is considerable. Anita Brookner has lots of single heroines but they are all sad and out of it. Barbara Pym has many single heroines but again, they tend to lead restricted mingy boiled-egg-for-dinner lives. They would definitely not inspire students to think a woman can have a big fulfilling life without a man!