Eye of the Needle (1978)
by Ken Follett
This is a book my father read. I saw the 1981 movie so the story was already familiar. Basically, during World War II the Nazis order their best spy in Britain to observe troop concentrations prior to the Allied invasion of Europe, with the goal of determining where the Allies are likely to land. This master German spy, "The Needle," is as cunning as he is ruthless, and he uncovers a secret that can change the course of the Second World War.
I came to this book after reading World Without End, Ken Follett's grand, somewhat annoying historical epic. Eye of the Needle is a much better book. Much shorter, too. Ken Follett was never a great writer, but no matter. Here he has a great spy story to tell. The tale is compelling, the characters are interesting, and the pacing is taut. The story is quite plausible and, as the author notes, it's hard to believe such a thing didn't happen!
Note; My brief book review of Ken Follett's World Without End is here.