here's some basic info on the subject, ghinger:
you must contact either the author or the publisher... and be prepared to pay at least 4-5 figures for it...
unless, of course, it's a self-published thing and not selling well, in which case the author may be willing to cut a deal with payment on the back end [if there ever is one]...
generally, no author/publisher of a book that's selling well will be interested in entrusting it to an unknown screenwriter newbie, so if you don't have a good track record, you won't get far with them...
if you have an agent, have her/him approach the current owner of the copyright and explore deal options...
if you don't have any screenwriting experience, i strongly advise you to learn the craft first... it takes years to learn the basics and get good enough at it to have even a chance to sell a script...
if you need any more info or want help, feel free to contact me... i mentor lots of aspiring screenwriters, as well as novelists and writers of anything else that takes words...
love and hugs, maia
maia3maia@hotmail.com