Because we started discussing this on the Caturday thread.
A friend at work recently gave me a stack of Karin Slaughter books. I was a bit suss about them, because I wasn't sure if I'd like them. I thought they might be "chick lit", but so far I really like this first one I've started on.
I've started with Blindsighted thinking it was her first one, but there seems to be some previous material hinted at in this book. Does anyone know the sequence? Does it matter?
Like in the Jack Reacher series by Lee Child? You can pretty much pick up any book and the past history will be revealed in part in all his books.
Love Jack Reacher 
If you've got a Kindle, there is a plethora of "post-apocalyptic" books on Amazon..........I've got more than I can ever get through, but they're usually free.......
I'm also quite enamoured with the Stuart Woods novels, he has three protagonist, who frequently appear in each others' series.
Yet another series that I enjoyed (at least the first dozen books) was the "Casca" novels, written by Ssgt Barry Sadler of "Ballad of the Green Berets" fame. The books are about a Roman legionnaire present at the crucifixion of Jesus, who is condemned to eternal life by Christ. Each book is a specific time in history, and they are fascinating. Unfortunately, Sadler died of complications from a gunshot wound to the head in Guatemala. His later books were ghost-written, and the writing style suffered.
They can be obtained at: http://www.casca.net/
