The Remains of the Day

Two adaptations that do an excellent job of keeping the same feel as the source:

Good Omens- series (season 1)

A Room with a View- older movie

Slow horses

Forrest Gump the movie vs. Forrest Gump the book.
I saw the movie first, then years later I read the book. The book is quite different from the movie. Same premise, but Forrest has different adventures in the book.

Never Let Me Go

Atonement

My Brilliant Friend

Poldark while they actually followed the books. The had to wrap the series up by going away from the books and that didn't go well.

Miss Fisher's Murder Mysteries (set in 1920s Australia) was an great adaption of the Phyrne Fisher books. They used some of the same stories, but changed them enough that the episodes always felt interesting. That show was perfectly cast. The costumes were great, too.

Big Little Lies (only saw the first season)
The last thing he told me