If I like the movie I probably will like the book.. But invariably the book is better and the screen writer gets it wrong.
With very few exceptions, Stephen King’s books seem to have been impossible to put on the big screen, but many tried any way, and failed. Of course, starting with Tommyknockers, King’s books became unsustainable from the get go.
In Harm’s Way follows the book closely, but the book fleshes out things the movie didn’t have time for. Anyone who likes that movie should read the book.
Just the opposite, of course, was 2001: A Space Odyssey. The thirst for interpreting the movie led to the book being written after the movie, and the book, being based on the screenplay was bereft of added character or plot development and a waste of paper.
The book for 2001 was mostly based on earlier stories written by Clarke and written down in full DURING the making of the movie; not afterwards, as you claimed.