Way back in the day, well before movies were very popular many people read books as a form of entertainment. In the 21st century though, people have gone from reading book to watching movies. There are many different types of movie genres out there, just as there are many different book genres out there. There has been more of a shift of turning books into movies.
There are many examples of books into movies such as John Green's "The Fault in Our Stars" this book by John Green has been considered a New York Times Bestseller and it has sold thousands of copies; the book is also being turned into a movie in the summer of 2014. Another book that is being turned into a movie is Veronica Roth's "Divergent". Another book that has gone from a book to a movie is F. Scott Fitzgerald's "The Great Gatsby" which recently was turned into a movie in 2013. This is a key concept with some authors because it is interesting to see how a book that one has written could possibly be visibly seen on the silver screen. With a book to movie adaptation a writer can really explore the in depth world that the characters are living and experiencing the world in.
One of the problems with book to movie adaptations is trying to fit: for example a 300 page novel into a two hour movie. Some of these limitations causes the movie to leave out certain characters or to change certain scenes around to fit the style of the movie. Many avid book readers feel upset sometimes when they watch a movie that is based off of a book and then they realize that the movie is a lot different than the book the movie is based upon. Overall, the saying that "the book is better than the movie" is true in most cases, but not in all.