Here are some off the top of my head I can remember liking:

Day by Day Armageddon: The first book is amazing and it trails from there.

The Stain Trilogy: Again the first one is awesome, then they fall fast to the point I stopped reading the third one for a couple weeks, before I finished.

The Passage: A Novel This is a huge book, that spans a large timeline. Like most modern "huge" books there is some slow reading and some boring parts but over all I liked it, there is a second one, but I haven't started it yet.

I know some will think the Stain and Passage don't belong here due to the fact they are not zombies, but if you want stories about monsters that eat people and society collapses, then that's what they offer.

LZR-1143: Within, and LZR-1143: Perspectives are two free short stories that are intros (bait) for the real series of LZR-1143 books. I just finished Within last night and after reading the two free books, the rest of the LZR series will be next.

Sawkill Stories: The Silver Tower This is another free intro book, I liked the story and the writing style, and will get around to reading the main book, I hesitate because there are plenty of typos in the free book, and the reviews of the main state the same thing.