Quicksilver in the Hand by Heather R. Blair

When her ex-husband disappears, Stacia drops everything to find out why. It doesn’t matter what happened between them or that Lucjan isn’t technically her ex. It doesn’t even matter that saving him will mean reentering Lucjan’s dark and dangerous world, where guns, drugs, and flesh are the currency of choice and the bratva lurks around every corner.
What does matter is the price they’ll both pay. Because in this world, the past always catches up with you.
And debts are paid in blood.