What Would You Recommend? - Think Of A Number

This is a question I get asked all the time working in the bookstore: "What would you recommend for somebody who loved "_________"? (fill in the blank)" Usually I can come up with something, but that something can be a bit of a wild guess if it's not a book or genre I normally read. This is where you helpful people come in. If you have a suggestion, I'd love to hear it.

Think of a Number is just flying off the shelves these days. In my mind it's a combination of mystery and thriller, but I don't normally read either. To be honest, I was asked to read Think of a Number at work last week, and I'm still working my way through the book. I am quite enjoying it though.

What I need to know now though, is what people who liked this book might like now. I'm already getting quite a few people saying they've already read Think of a Number and they really liked it, when I recommend it. I'm guessing that there might be some suggestions out in the blogsphere given the comments on my last It's Monday! What Are You Reading? post. Apparently, the book has been doing the rounds of the blogs in recent weeks.

Given that this is John Verdon's first book, I can't exactly recommend any of his other books yet :), so what would you recommend in this case?