The book is surprisingly different, including trips to Beirut and a Pacific island to watch the testing of a nuclear bomb. Harry Palmer isn't even in it, as the anonymous narrator tells us on page 34:
"Now my name isn't Harry, but in this business it's hard to remember whether it ever had been."
In the film, Palmer is played by Michael Caine, a Londoner born in 1933. Not-Harry in the book is from Burnley - completely changing how he'd sound - and perhaps a decade older, as we're told he was in the fifth form in 1939.
Despite the excursions abroad, the plot is basically the same, with the same mix of drab bureaucracy and imminent danger of death. There's the brilliant twist when the agent escapes from incarceration and discovers it's not been quite what it seems - which is so good I don't want to spoil it here, nearly 60 years after the book was first published.
But my general feeling is that the book is a poor relation to the film. The screenplay condenses the story, reducing the scale but making more focused, quicker-moving and sharper. Even minor characters in the movie are memorable - such as Tony Caunter's non-speaking American agent, a big guy with a distinctive glasses, a plaster over the bridge of the nose. The two men who stand out, I think, are the ones who are kind to our narrator in his hour of need. (He makes sure to pay them for their kindness.)
So I'm a bit surprised by the cover line on my battered second-hand copy of the book from 1995 the Sunday Times calling Deighton, "The poet of the spy story." Surely that's a better description of le Carre, whose prose is so much more beautiful than this clunky stuff. It's fine, it's fun enough, it's got some great moments... But the film is witty, stylish, and so classy that it holds its own against Bond.