The River Queen (Book 1 in the Stonemaw Chronicles #1) by TM Mayfield
 
“Prophecies have long been spoken of a mermaid queen who will lead the kingdom of Rivenden into its greatest age…”

When unexpectedly faced with running the kingdom alone, Queen Adara of the Rivenden mermaids quickly finds herself facing a foe she never expected to see again. As if that wasn't difficult enough, she finds herself falling for the one merman she shouldn't. All she wanted was to make King Adonis proud, but she now finds herself second guessing every step.

Commander Orion, known as the best of the best of Rivenden’s defenses. He has been tasked to protect Adara by his king and childhood friend, Adonis. After the unthinkable occurs, Orion finds it harder to not fall for the one mermaid his heart has desired since he laid eyes on her. After Adara informs him of an outside evil that is encroaching on their kingdom, he places himself at her side.

The journey that Adara and Orion embark on will not only test them as individuals, but also their strength, friendship, and trust in one another. But what happens when Fate deems it necessary to throw more at them than either of them could have planned for?