John Le Carre - Tinker Tailor Soldier Spy.
I put of reading this for a long time because I love the 2011 film adaptation so much. Silly of me to think the book would make me feel any less about the movie. While the novel is possibly the best spy thriller of all time so is the film one of the best adaptations ever. If anything I now like the movie more, it is so expertly done, fully adapting the novel, not being afraid to change things to make them more filmatic, or less internalized. All the while fully keeping to the heart of the novel, if not the letter. Especially the multiple timelines in the film (the book is quite straightforward) which have made it difficult to follow for lesser minds (sorry) are a genius invention to actually simplify the story.
The old BBC adaptation is impossible to watch here legally, might have to venture to the high seas for the first time in a long time to finally watch it.