Hearken (Daughters of the Sea Series) by Kristen Day

Persephone and Charon when many of the Gods and Goddesses banished to Tartarus kept escaping. They needed a prison they could be sure was impenetrable, as well as inescapable. They then built Tribeca beneath the Underworld. No one has ever escaped.”
                  “Where was the Key?” Avery asked quietly.
                  “No one knows,” Amphitrite said. “Which was the point, of course. The less people that knew, the less likely it would be stolen. Or…that was the thought process.”
                  “So what’s the Book of Souls?” Phoebe questioned her.
                  “The Book of Souls is just a ledger of all the souls admitted into the Underworld…” she raised an eyebrow, “…including Tribeca.”
                  “So the fact that they were stolen together means someone is looking for a specific God or Goddess they believe to be in Tribeca?” Avery surmised.
                  “I believe so,” Amphitrite nodded gravely. “But it is impossible to know which one.”
                  “How many Gods and Goddesses are imprisoned there?” Phoebe questioned.
                  “More than you can count,” Amphitrite revealed to her with a pat on the hand. “But I have a feeling the Sons will take care of it. If they need our help, they will ask. Until then, we must stay focused on our own responsibilities.”
     
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                  “I can promise you she’s not mad,” Phoebe pleaded with me behind puppy dog eyes. “Come with us and you’ll see!”
                  “Alright,” I agreed; guilt nipping away at my heart. I reluctantly settled into the back seat of the sleek black golf cart behind Carmen. “I hope you’re right. I feel so bad about the selection meeting. I really did have every intention of choosing her for Apothecary, but apparently it wasn’t up to me.”
                  “She’ll understand. I don’t think she’s even capable of holding a grudge.” Carmen brushed off my worries with a wiggle of her eyebrows. “She reserves those for all her boy-toys.”
                  “You mean all two of ‘em?” Phoebe giggled and peeked back at me with adventure shining in her eyes. “You better hold on!” She slammed her foot on the gas; plastering us against the hot leather of the seats.
                  “Lay off the gas, Earnhardt - you’re gonna rip the skin off our faces!” Carmen shrieked. She grabbed onto anything nailed down to avoid being tossed onto the same asphalt that flew beneath us at warp speed. I searched for a seatbelt that wasn’t there, and ended up settling for a death grip on the back of Carmen’s seat.
                  “It‘d be a lot more fun if it did,” I heard Phoebe mutter. The daredevil smile that slithered across her lips had me shaking my head and trying not to smile. 
                  “Where are we going again?” I called over the wind. 
                  “The east beach!” Carmen’s answer trailed off as we took a sharp curve too fast and almost made it up on two wheels. I hooked my toes under the seat, spread my arms out wide, and surrendered to the thrill ride. I was immortal, for goodness sake!  If one rogue golf cart trip could kill me, I didn’t deserve to be a Goddess.  I closed my eyes and squealed as we drove out of the gates of Lorelei, headed north, and rode up the east side of the island. Thick live oak trees etched out a tunnel surrounding the road, which beautifully cast an enchanting effect with rays of light filtering through the leaves. After several turns and two near head-on collisions, we finally hit a dead end and parked next to two more black Lorelei golf carts.
                  “We’ll have to hike it from here,” Carmen announced ominously. A scorching wind cropped up around me while the