There are 4 books in the Waterfire Saga series. The newest book is Sea Spell which came out in 2016.

1
Deep Blue

Book cover of Deep Blue

When Serafina, a mermaid of the Mediterranean Sea, awakens on the morning of her betrothal, her biggest worry should be about reuniting with handsome Prince Mahdi, her childhood crush. Instead she finds herself haunted by strange dreams foretelling the return of an ancient evil, and dealing with the deaths of…

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2
Rogue Wave

Book cover of Rogue Wave

Six mermaids battle to save their hidden world.

Serafina, Neela and four others have successfully escaped Captain Traho and his men after their hideout with the river witches was stormed. They must stop an ancient evil from escaping its icy prison by finding six talismans hidden by Serafina's ancestor hundreds…

3
Dark Tide

Book cover of Dark Tide

Six mermaids battle to save their hidden world.

Leaders of a resistance to the barbaric new regime, Serafina and Neela, are working hard to destabilise Vallerio, the new ruler of Cerulea. Becca, Ava and Ling are still hunting for their ancestors' talismans; the keys to keeping an ancient evil locked…

4
Sea Spell

Book cover of Sea Spell

Six mermaids battle to save their hidden world in Disney's epic series finale for fans of Percy Jackson and The Girl of Ink & Stars.

The mermaids prepare for war. The powerful Volneros is invading all the mer realms and in a foolish attempt to stop him, founding father Orfeo…