I will nominate Portrait of Jennie
by Robert Nathan.
In this gentle novel, Nathan uses the device of "time-slip" to create one of the most haunting, moving and beautiful romances ever written. Many are familiar with the love of two people separated in time from the wonderful film of the same name. In fact, the book is even better. Where the film did make changes, they were usually inferior to the original conception of the author.
It is available in ebook format from Amazon: