I'll nominate
Portrait of Jennie by Robert Nathan.
It's a gentle, beautiful, and moving time-slip romance between two people living in different eras.
The film with Joseph Cotton and Jennifer Jones is justly famous, but the book is even more engaging.
I nominated this once before but I believe it was more than 6 months ago.
It's available at Amazon UK here:
http://www.amazon.co.uk/s/ref=nb_sb_...rait+of+Jennie
and for Amazon.com
http://www.amazon.com/Portrait-Jenni...rait+of+Jennie