Locate contact information for the subsidiary rights department of the company that published the book in which you are interested. Often this information appears on the websites of individual book publishing companies. If the book is self-published, locate contact information for the author.
Send the publisher or author a fax, email or letter inquiring as to whether rights are available for the book. If the publisher or author has sold or optioned the rights already, you'll have to find another book to use for your film. If the book's copyright has expired, the book is in the public domain and you are free to adapt it into a screenplay or film without paying any fee or royalties.
Hire an agent or entertainment attorney if film rights are available. An experienced professional who knows both the book and film industries can help you negotiate the acquisition of film rights. He can also help you understand the legal jargon of contracts you'll need to read and sign.
Consider optioning the book instead of purchasing the rights. Optioning a book allows you to have exclusive film rights for a book, typically for a period of six to 12 months, and is considerably less expensive than outright purchase. You'll have the ability to renew your option after it expires, but you'll also have to pay an additional fee.