If you're serious about selling and/or producing the screenplay you're adapting, check to see if the film rights are available before spending a great deal of time on the writing process. The odds of getting the rights can be slim, depending on the type and popularity of the book, unless you have an agent or connections in the film industry.
Find the contact information for the publisher and its auxiliary/subsidiary rights department. Go to the publisher's website; most of the major ones, such as HarperCollins and Simon & Schuster have a link to the page that explains how and where to inquire about the rights. Or, basic contact information for the publisher can sometimes be found in the front or back of the book.
Use the same methods to try and contact the author directly. An author may be receptive to granting an option on the rights--an arrangement where the terms of the rights fees can be determined after the screenplay is finished and a deal is made.
Writing a screenplay, as with any form of writing, is a personal process--every writer has a method and rhythm to shaping the story and reaching the finished work. Therefore, there isn't one definitive way to adapt a book into a script; the best thing to do is find the method that works best for you. That said, there are a few basic steps you can take to make the process easier. Books and films are two different means of storytelling--what works in one may not work in the other. Find what it is about the book that would make a good movie and start there.
Don't set out to do a literal translation of the book to script form. The rule of thumb is that one page of a screenplay equals one minute of screen time, so with most novels, keeping everything from the book would mean creating a movie that would be far too long. Film is a visual medium--showing, not telling. Use the book as a starting point and not a complete outline. If it's a nonfiction book, such as a biography, showing every event in that life would take too long; decide what parts fit the story you want to tell, and find a good beginning and end point.
Read through the book a few times to get a good grasp of the tone and characters, then make note cards listing the key scenes and elements. Use those to pare down the story, filter out what isn't essential and find the shortest, most direct path from the beginning to the end.
Most screenplays feature a three-act structure--a beginning, middle and end. In the first, characters and plot are established and the story is set in motion. The second builds the conflicts and creates a turning point that leads to the third, where character arcs and conflicts reach a satisfying conclusion. Break your story down into these segments to figure out where it needs to be at each stage.
Don't be afraid to deviate from the source material. Characters can be added, dropped or combined, the order of scenes can be cut or rearranged; even the ending can be changed to fit the story you need to tell. Again, the book is just a starting point, not the basis for everything that ends up in the script.
Write the first draft using the outlines you've created. The most efficient way is with the use of a screenwriting software such as Final Draft, which places the specific elements (headings, action and dialogue) into the proper format. Flesh out your story from beginning to end, using basic descriptions for the action; details are filled in by the camera during filming.
When the first draft is done, you'll have a good idea of where to begin the rewriting process, based on length and how well the story flows. The average script is around 120 pages, enough for a two-hour movie. Keep rewriting until you're satisfied that the story is ready to be filmed.