1. No matter how long between books Barry goes, reading John Rain is like catching up with an old friend.
2. He has a really good/heinous villain in Zero Sum. There's even a scene where you feel a little sorry for this bad guy!
3. There are elements that make me think of Lee Child and Harlan Coben. Doesn't get much better than that!
4. There are surprises. I like a book that you can't figure out ahead of time (of course, that is my M.O.)
5. It is very well-written.
(6. At one point, Rain is comparing his actions to an army after-action report: "What did we do well, what could we have done better, what can we learn to improve the odds next time." This is exactly what Joni used to do after an event when she and her partner had their event-planning business!)
So if I took so many positives from this book, why was I disappointed? The answer is simple: There was too much graphic violence. Now, violence per se does not bother me. The problem here is that the older John Rain (Zero Sum takes place as one of several prequels to the 1st group of books) was an assassin who used means that were designed to NOT look like a murder. Every killing was planned to be very low-key. The killings in this book were extremely brutal. It was just too much for me based on what I was used to. If you don't mind that, or if you haven't read other John Rain stories, then you will be fine with Zero Sum. Most importantly, under no circumstances will this prevent me from reading any book that Barry puts out there.