The monsters are zombies. The island is Manhattan, which is kind of like a giant roach motel: the zombies check in, but they don't check out...that is, unless they are shot through the brain, decapitated, or otherwise have brain functions terminated.
Dekalb, a former UN weapons inspector who was caught in Somalia when the shit hit the fan is on a mission. He and his young daughter were lucky enough to be taken in by a group of girl soldiers who serve a female warlord. The price of Dekalb and his daughter's continued well-being hinges on Dekalb's ability to find the drugs the warlord needs to treat her AIDS. Unfortunately, all of the UN outposts in Africa Dekalb knows of have long since been looted. So Dekalb and a team of girl-soldiers set sail on a commandeered ship to the only place in the world Dekalb is sure there will be a supply of the necessary medicines.
New York, when they get there, is every bit as bad as they could have imagined. Not only were the zombies unable to check out, neither were the living. It's a mess, and the odds are against them, but they set out in search of the drugs. Along the way Dekalb and the girls meet up with Gary, who's dead. Or Dead, I guess. Gary, who had been a medical student--although he tells people he was a doctor--saw the writing on the wall and killed himself after ensuring that his body and brain would be properly preserved. Consequently, unlike his Dead brothers and sisters Gary is able to think and speak.
For a while he's even on the side of the Living, but things change.
Monster Island, originally published serially online, is a lively take on the zombie mythology. Although the origins of the plague are apparently of the usual biological sort (although I haven't read Monster Nation, the second book in the series which is a prequel explaining the plague's origins), somewhere around the middle of the book it takes a turn for the supernatural, and it ends on a bizarre and disturbing note. All in all, a fine entry in the zombie canon.