Whatever movie is based on a book, reading the book before seeing the movie is a good idea. There are advantages and disadvantages to both situations, and many factors depend on the genre. However, there are reasons why it is always best to read the book first.
The book has more detail.
Because movies usually have time constraints, the book will have more detail about a story than the movie does. This means that reading the book in advance will give you the details of the story that the movie needs to cut for sake of time
You will understand the story background better.
The author of the book often provides information about events leading up to the story. For example in a horror movie, the teenagers might get marooned at a spooky house, but the book may give the reason for the trip.
You will understand the characters better.
By reading the book, you will usually understand the motivations of the characters a lot better than you will from a movie. For example, the book may give you an extensive background of the antagonist of a horror story and why he is a psychopathic murder out to kill the teenage girls. A movie might not include that information.
You will understand the movie better.
Finally, the book gives you a lot of background information that actually makes it easier to understand the movie. Often movies are about getting the main plot as quickly as possible because of time constraints none of which exist in a book.
While reading the book in advance can ruin a surprise ending, more often than not, knowing the ending in advance does not hinder your enjoyment of the movie — it can actually enhance it. So, if you have the opportunity, read the book before going to the movie with horror stories.