Marilyn Smith Yes. Absolutely. There is a progression with each novel, and reading them out of order would diminish your understanding. This is true of the Allon series more than almost any series I've ever read, and I tend to read all of an author's work. The characters grow and age, and one novel builds upon another.
Colleen Reynolds Thank you for your answers. I did decide to read this one (because it was free on KindleUnlimited) and so far, so good. I will admit, it gets a little confusing understanding the names of all the foreigners and where they stand in the story. I'm having a little difficulty establishing how some of the relationships have developed. I think once I get through this, I may go find the first and second ones.
Jesteen Schaetz Not to understand the main plot. I didn’t know it was part of a series when I started reading it, but I got through it fine. There were certain parts when I realized there had to be other books (just too many references to the past, it was obviously more than backstory), but it was easy to infer. Perhaps there were little nuance things I missed, but i doubt it was too significant. The only reason I can think not to read it as a standalone is if you plan on reading the other Gabriel Allon books and don’t want spoilers, they do make reference to several other missions and how they turned out, which I'm sure must be the story lines in the other books.
Christine No, you don't have to read them in order, but it helps to understand the characters and how far they've come in their personal journeys.