This is a spy novel, not a thriller, and there is a real difference between the two genres. Think John LeCarre and Graham Greene, not Robert Ludlum and Ken Follet. With the spy novel, you have the ever-so-slow peeling of layers, deeper characterizaion, a frequent sense of foreboding and, until all is revealed, some confusion. The thriller, in contrast, is the page-turning, up-all-night, action-packed adventure that you can't put down. After finishing a thriller, you are likely to say "where can I get another fix," but not to reflect on what you have just read, and if you try, you may not remember and, if you do, it may not make sense. With the spy novel, you may want to wait a while before reading another, but you will spend some time reflecting on what you've just read, and it provokes you in a more serious, literary way.
I like both genres but find it important to orient my expectations going in.
For the spy novel genre, Restless would have to rank among my favorites. In addition to the terrific writing, the likeable-but-far-from-perfect heroines and the World War II intrigue, the novel offers some additional pleasures.
First, it is quintissentially British. The book involves, among other things, a single mother raising her son, the world of Oxford academia, and all sorts of emotionally powerful events. These all come across with the British stoicism, stiff-upper-lipism and "no winging (whining)" ethic that make the book very different from an American treatment of the identical plot. Not better, or worse, just different and thus very interesting to the American reader. The cultural difference (accurately renedered I should say) is a fascinating sidelight for the American reader.
Second, the author employed heroines rather than heroes. I would be interested to hear from female readers, but I was very impressed with the author's ability to create characters of the opposite sex who seemed nonstereotyped, but true. There is nothing of "the weaker sex" to the heroines, but they are not at all the same as they would be if written as men. In short, they're real women (or at least seem so from my, male, perspective)in a genre that does not frequently offer that.
Third, the novel spends a great deal of time on the intrigue, spying and propoganda surrounding British efforts to persuade the United States to join World War II. In an interview, Boyd says that he mostly used his imagination in creating the spying, but it certainly seems realistic and oh so relevant today. The wheels-within-wheels manipulation of the media and public opinion and the "trust nobody" mantra say more about contemporary foreign affairs than many current nonfiction treatments, which themselves simply repeat the spin that interested actors have given the authors.