Book is better than the series, as usual. However both though are not set in reality, the characters, plot twists, money are outrageous. It's a fiction mystery set on a beautiful island. The story doesn't need to "realistic" to be a great summer mystery.
The series uses the book as the overall guidance but takes liberties with time lines, key moments, non essential characters and actually offers an ending where someone is held accountable.
Overall I liked it, the characters are outlandish, story is twisty, backdrop is beautiful. Recommend both TV and book but my preference is the book.