Still She Wished for Company — Margaret Irwin

First published in 1924, now public domain in the US and Canada. The story moves between the 1920s and the 1770s, following two heroines: 20th century Jan Challard, a London girl, and 18th century Juliana Clare, the youngest daughter of an aristocratic Berkshire family. Jan is independent and spirited, but leads a humdrum life, working in an office, and walks out with a very suitable young man. Juliana, at 17 years of age, is getting the upbringing of a young lady in the enormous family mansion, Chidleigh, and her life is devoid of excitement and event.

The two heroines can see one another through some rent in the fabric of time, but never manage to meet and interact. Their lives converge as Juliana's world is turned upside down; her father dies and her notoriously wicked and mysterious brother, Lucian Clare, returns to take his position as head of the family. Lucian recognizes a supernatural power in Juliana, and uses this to reach out to Jan through the ages to unite in time from different time periods.