Graham Greene -
I remember laughing to tears while reading
Our Man in Havana 
I don't think I read
The Third Man, the movie was so great, I didn't feel like reading another version. Maybe it's time I did...
Umberto Eco - oh yes! Though I think my favorite was
The Island of the Day Before (I hope that is the correct English title). I remember the mention of the mountains of iron that must be at the North Pole, to explain why the compass needle pointed North.
And thinking about Eco also puts to mind
An Instance of the Finger Post, by Iain Pears. Very different, but another description of how learned men of past centuries approached science and the pursuit of knowledge.
I would give
An Instance of the Finger Post and
The Island of the Day Before at least a 4.5. I always hesitate to give the absolute best rating to a book though, so I have a lot of 4.5 on LT and very few 5.