The Singing Towers of Darillium

This story is the one mentioned by River Song as she spoke to the 10th Doctor in Silence in the Library/Forests of the Dead but actually takes place with the 11th Doctor at the time of the short feature "Last Night" with a bit of reference to "First Night". It may seem to wax poetic about River Song, but that is only the surface. It's actually tale of what it is to love a mortal.