I read last year a book about a high lord elf whose job is performing magic; he loves books and is looking for old books containing old magic, so called sexual magic.
He finds an intelligent, beautiful human woman and falls for her, but he uses her in order to perform the magic. She solves the mystery of fertility for the elvin population; it is to not perform magic for a month. This secret was lost by the priests, although the hero's father knew it but, because he hated his son, he never told him.
The hero is elected emperor, after intense adventures and his human wife, pregnant, becomes empress. The society in which they live is composed of humans, who are the lowest socially, and different mixes of elves/humans. Pretty good book.
I can't remember the title or author; thanks for helping.