The Separation - The Separation Trilogy, #1

In a world where men and women live on separate sides of a massive wall, seventeen-year-old misfit Eroyn Fairchild has always been too busy with her broken family to wonder why they live the way they do. When a man from the other side breaks through, Ero holds him hostage, hoping for a ransom large enough to pay for her Elder Grace's treatment. Things get more complicated as the man is followed by two others who make Ero question everything she's ever known about her life.
As Ero searches for the truth, the lines between right and wrong blur, leaving her to choose between saving her city and saving herself.