I recently finished reading the last book in David Moody's Autumn series. It's a six book series in which billions of people die of some sudden on-set disease. Those who are immune must somehow survive, but survival becomes more difficult when the dead rise and start to move around. As each day goes body the bodies continue to decay, but brain activity increases, making them smarter and more dangerous. In other words it's a zombie series, although the author never uses the word 'zombie'.
I love zombie books. Yes, I know zombies aren't real. I really hope you didn't think I thought that. :-) I like to read science fiction/fantasy books that have a touch of science, but then go beyond that. There's little science with zombies, but these books have some truth in that they discuss the physical and psychological effects on individuals placed in a do or die situations. One never knows how they'll react until placed in a very difficult situation in which death is a very real possibility. If you like zombie books, you can't go wrong with David Moody's Autumn series.