Check out freebooksy.com and bookbub.com. At least once a week they have at least one zombie book listed for free. I haven't read it yet, The Zombie Chronicals, Fat Vampire, The Secret Apocalypse, Zombie Fallout, The Dead, and the Da Vinci Code. Occasionally you get an awesome hit when they find a big name author or a popular title. (I read the Da Vinci Code some time ago and have it in hard back.)