In terms of books that have made it big in being allowed to grace the silver screen, two titles jump out immediately. The "Harry Potter" series and "The Lord of the Rings" have without a doubt become two of the largest household names throughout the world. For all their success, though, they are exceptions to the rule when it comes to making a movie from a book.
There are many benefits to making a book into a movie, not the least of which is to please those who follow the series. In granting the dream of being able to see their favorite characters and locations come to life, diehard fans and casual observers alike get the chance to watch and experience the book as the author does, up close and personal. The opportunity to see the inner workings of an author's mind can be quite intimidating as well as thrilling at times. What is unfortunate about this is that quite often, what is seen on the big screen is not what was intended.
In order to get within sight of the silver screen, a book must be optioned by a producer, which means they first buy the rights and then make plans to begin putting together the many elements needed for the film. Many upon many books never make this leap, as they are either never noticed or are not deemed fulfilling enough to attract even the most dedicated fan. Those that are selected are often stymied for a great deal of time as producers, directors, and other vital components of the filming process are needed to get a film rolling.
Aside from the technical pitfalls of being placed into film history, the content of many books is often changed to accommodate the vision of a director. An author does not often have final say over what appears in a movie based upon their book, and thus their main points and finer details can be lost. In "Harry Potter" and "The Lord of the Rings" there are many such discrepancies that loyal fans have noted. This has been largely forgiven by many fans who are simply enamored of the film series, though it is a downfall of the transition from print to film.
Added to this is the very real possibility of poor actors being cast for roles that do not fit them, dialogue that wouldn't challenge a young child, and even budget limitations that do not allow for certain effects and scenes to be included. The whittling-down process of any book to film transition is an uncertain and often frustrating ordeal that can ruin an epic story or make it legendary. Authors are thought to be highly compensated when a producer shows interest in their book, but the risk of ruining their vision and their reputation is very real. There are benefits to turning a book into a movie. The author gets paid, and the fans get to see their favorite stories come to life.