World War Z- a 'history' of a zombie apocalypse. Good read.
The Anita Blake: Vampire Hunter series by Laurell Hamilton. Though it gets into a bunch of raunchy sex stuff by the 5th book or so. Vamp Wife told me it got better after that, but haven't read them yet.
Zombie Chronicles by James Melzer. Free downloads from his site Jamesmelzer.net. Nice story, different angle.
Necroscope Saga by Brian Lumley. Check his website for the order of the books, they don't go 1, 2, 3, etc. I liked these, though I only have read the first 5.
The 'Dark' series by Christine Feehan. Not a 'tween series, but a big focus on love. Actually, it's weird. Lots of violence, though.
There's always Lovecraft. I like me some Lovecraft, read every story.
There is a series of Resident Evil books that are side stories from the games and movies. They are OK and quick reads.
Some more info on some sites you can check out:
Here is a website with a bunch of zombie books listed.
Here is a website for vampire books.
Some on post apocalyptic books.