Assassin's Code: A Joe Ledger Novel is the first of Jonathan Maberry's Joe Ledger novels that I've read. I do have a copy of Patient Zero on my Kindle, but I haven't gotten to it yet. Being unfamiliar with the backstory did make things a little confusing, but much of it is hinted at in the book itself.
From what I gather, Joe Ledger is a commando, or something of that nature, who works for a secret government organization called the DMS. He and his team are in Iran, where they have just rescued a kidnapped group of American Hikers. Afterwards, Joe is forced to meet with an Iranian official who ask Joe to find some stolen nukes. Unfortunately, it turns out that the nukes are in the hands of a cult of vampires.
This leads to a lot of fighting, and shooting, and people getting their throats torn out, which is all very satisfying in a thriller. It also leads to a heck of a lot of phone calls being made. At some points it seemed like Joe would either be calling someone or getting a phone call from someone just about every other page. This slowed down the pace a good bit, but didn't detract from the overall story too much. It's still a very thrilling book.