The problem with having your heroine hypnotized and in the psychic thrall of another character for most of the book is that said heroine never gets a chance to really grow as a person, or show any gumption or initiative. It's kind of boring. This was what kept me from enjoying Still She Wished For Company, by Margaret Irwin (1924).
But it is not terrible, and if you like tortured romance stretched across a century of psychic connection, with most of the action taking place in a Jane Austin-ish country house, and lots and lots of growing dread and even horror, and don't mind a heroine who is being used as a tool, you might enjoy it. Just to emphasize that others have enjoyed it lots, here is a positive review at Vulpes Libris.
Juliana is the youngest daughter of a wealthy family, living a boring, sheltered life of late 18th-century placidity. Into her world comes her long-gone oldest brother, Lucian, who has been dissipating himself all around the world, and he brings Interest to things with a vengeance. Lucian is haunted by a girl he has never met....and it turns out that this girl, Jan, is haunted by him as well--though she lives in the mid-20th century...and it is this that drives him to make Bad Choices.
Lucian makes a pet of Juliana, and she finds him a charming alternative to boredom....and their relationship is oh so warm and friendly and one wonders is it getting too friendly (there is an ickiness to his endearments) and then Lucian draws her into mystic goings on of a disquieting sort and Juliana starts slipping into Jan's present time and Lucian gets creepier and in the end...What will become of poor hypnotized Juliana? Is Lucian demon or tormented romantic? Will Jan find happiness, and if so, with whom? The book actually starts from Jan's perspective, which is a bit of a red-herring--she does a bit of timeslipping herself, but primarily exists in the story to be Lucian's dream girl, and this made me sad, because I rather liked her and we see very little of her.
In any event, this is the earliest tortured paranormal romance involving time slippage that I know of, and as such it is interesting and well worth reading (I guess). If there are earlier ones, please let me know!