FREE: Shadow Zone by Iris Johansen

Back of the book says: Deep below the Atlantic Ocean, guarded by hundreds of dolphins, lie the astonishing remains of an Atlantis-like lost city called Marinth. Its disappearance has confounded scientists for years--until now. Marine architect Hannah Bryson has unearthed a legendary artifact that might hold the key to unlocking the city's long buried secrets. But Hannah's discovery could have dire consequences for the modern-day world--and there are some people who will do anything to keep the truth about Marinth hidden...
When the artifact is hijacked, Hanna stands alone between a power-hungry enemy and widespread destruction. Willing to form whatever alliance is necessary to regain control of this potentially world-changing weapon, Hanna joins forces with Nicholas Kirov, a loose cannon from her past who is now her best hope for survival. Together they will race to unravel Marinth's last great secret...and before the lethal game is over, Hanna will realize that nothing and no one are what she expected--not even the dolphins, who know so much more than anyone could imagine...