Sirena is a siren, a mermaid. She and her sisters sing songs to lure sailors in, because they have been handed a promise (and curse) that if a man should love them they would become immortal. Irena grows disgusted at love born from trickery and seduction and seeks a love that she can earn through more honest means. And when she finds it and the immortality it brings, it is not quite what she expects.
This is an extremely ambitious and powerful book, albeit a bit uneven. The book works best when Sirena reveals her adolescent curiosity and longing for love. It works less well when it tries to reach some sort of deep poetic meaning. There are sections that bring tears to the eyes or flush the cheeks, but there are also long interludes that drag on painfully. The story is a stronger concept than narrative. And it is not really a YA book. This doesn't mean that young readers would not enjoy it, but this is a story about grown up love and passion in the long run, and seems more centered on that world.
I rather suspect that most younger readers would find it a bit dull.