Rob W. Hart - Free Library Land Online

It's been two years since the outbreak of a plague that turned New Yorkers into flesh-eating corpses. The city’s population has dwindled to three hundred refugees on Governors Island, a park and former military outpost. They struggle with supply shortages and flaring tempers, but the monsters they call ‘rotters’ can’t swim. The survivors aren’t comfortable, but they’re safe.
That sense of safety is shattered when Sarge, a former cop and the island’s head of security, comes face to decomposing face with a rotter while on an early-morning patrol. The creature shouldn’t be on the island. What’s worse is that it has a stone-like skin that makes it tougher to kill.
Faced with the prospect of an evolving enemy, and desperate to find antibiotics for his dying wife, Sarge has one option: Get into Manhattan, do some recon, forage for supplies, and get out—without drawing the attention of the millions of rotters that now roam the city.