Almost half of the movies which are made each year . . . are adaptations. The reason probably is because movie-makers hate to take risks. If a book has sold millions of copies they know there's an audience that will watch the adaptation no matter how bad it is. So supposing you've heard that a book will be adapted for the screen would you want to watch the movie first before reading the book or read the book first before watching the movie so you know what to expect?