I have forgotten the title of a book I read around 10 years ago. The book was fairly old then. The setting was in times of horses, carriages and telegrams. It consisted of a gentleman who went to stay as a live-in teacher for 2 children and fell for their mother. The father had traveled a lot and became disabled (due to a shooting incident or something) and used to drink a lot. He generally wasn't a nice guy.
There was also a cleaning maid (who helped to hide the affair) named Horton. And a family doctor who used to come visit. I think the man (teacher) used to write to his sister named Aggy or Agatha. Eventually the wife kills the husband (by poisoning or some sort). Some lines I remember and can paraphrase:
No doubt find Horton stretched across the threshold.
The doctor is blowing smoke, and he is fanning it this way.
This was during pre-Google times, and all search terms I used did not produce any close results.