My impression is that Bridges of Madison County is reasonably faithful to the book.
In fact, NSM. My feeling on both (as a consumer of both) is that the movie (written, amusingly, by the screenwriter of The Ref) is deliberately, fiercely unfaithful to the book.
The book really has one character, the Handsome Stranger With The Magic Cock Who's Gotta Keep Moving On Because He's Free Like That, Baby, and You Wouldn't Love Him Any Other Way (who also has a sorrowful lost love that haunts him, a rich and astonishing childhood far in the misty past, twice the photographic talent of Stieglitz and Avedon put together, and such depth as a jazz and blues musician that Magical Negroes break down and weep talking about the blinding beauty of having gigged with him--my hand to God, I'm not making the last bit up).
It also contains one cipher, an Italian war bride with no discernible personality traits aside from being Italian and a war bride (traits which are told, not shown), and a bunch of cardboard cutouts roughly indicating the Dull Husband With A Cock That Doesn't Sparkle, The Kids, The Censorious Neighbor, and the Other Townspeople.
She loves him because he is the biggest, most brilliant, most erotic, most talented violet-eyedest Marty Stu in the history of lousy books, and he loves her because...uh. Because she's there to be loved, a nice Bella-like blank into which every reader can insert herself. It's an incredibly shitty book, and a faithful movie would have sucked rocks. But the unfaithful movie gave the war bride a backstory and an inner life, made her cardboard family more vivid and fleshy, and kicked Marty in the nuts over and over.
I definitely know a few people who hated the movie precisely because they'd loved the book (or loved the movie because they'd hated the book) and it was obvious to them that the movie stopped just short of openly saying, "Hey, Book? FUCK YOU, YOU FUCKING FUCK."