Books make great source material for a film. There's a built-in audience, especially if it's a best-seller. And built-in audiences equal interest from financiers as it helps them to feel confident that there will be butts in seats when the film releases.
So, as a filmmaker, how do you go about securing the rights to make a film based on the book you have found? That's easy. First, you need to track down who represents the rights to the book. Typically, a literary agent will represent the book rights.
My first stop for figuring out who represents the rights to a book is the publisher. I contact the publisher and ask who the agent for the book is. Now, this can take a while and sometimes the publishers won't even respond, though I have found most publishers to respond in a timely manner. A film based on a book helps book sales so they are motivated to have a book adapted into a film.
If I can't figure out who represents the book from the publisher, I may Google the author and see if he or she has an Internet presence. If so, I will write the author and express an interest in his/her book and ask him/her for the contact information to the literary agent.
I will be honest here. Agents do not like it when you contact the author directly. But, if you have no other choice, you do it. And that personal touch with the author may help you win the option.
Once I am in touch with the literary agent, he or she is going to want to know if you have the money to option the material. And he or she is going to want an offer. Whether it's a dollar or $10k, they want an offer. I have not found any agent to ever quote me a figure. I am certain you won't get very far if your offer is a dollar and if that is all you can afford, you may want to stick to speaking with the author directly... I didn't say that!
So you made your offer and it was accepted. Now you need to draft an option agreement. It's best to work with an entertainment attorney on drafting a book option. They will help you craft the terms for the book and determine a proper purchase price. If you are too poor for an entertainment attorney, I am sure there are basic book option templates on the Web or you could ask a colleague for a copy of one of theirs and craft your own. I don't recommend this course of action, but we're all indie filmmakers here and we know sometimes, you just have to do what isn't recommended all in the name of making movies!