Here's some quick pros & cons concerning adaptations. PRO: You don't have to make up the story - it's ALL there. CON: In fact, that's the rub with adaptations, especially from novels. There's often TOO MUCH story. Novels sprawl, like Mexico City, they take up and infinite amount of space, just because they can. Especially some of the genre novels, like family sagas or historical fiction (Ken Follet - World Without End almost was). It's practically genetic that these babies clock in at 500 pages or more. Whereas, screenplays are more like Venice; self-contained and restricted to a very particular area because of limitations put on their size.
PRO: Producers often like the idea of adaptations because they feel the book or comic book brings with it an automatic audience and it's popularity is a harbinger for the film's eventual success. CON: If yo loved the book and thought it would make a great screenplay - there's a chance other screenwriters thought so too. If you're talking about a best seller; the rights will NOT be easy. Some agent is going to want some serious coinage for the rights to a best seller or even a mediocre seller if it comes from a big publisher.
What about the rights? Well, I always advise, WRITE WHAT YOU LOVE. So if you are deeply, deeply smitten; then break down the story, make your story choices and start writing. I also always say 'No writing experience is ever wasted.' At least yo will learn just how challenging in it's own way an adaptation can be for a screenwriter. At the very least, by the end of it, you'll have a 'spec' adaptation for your writing portfolio. But beware, on the way to completion if you fall madly, madly in love with the script and cannot live unless you acquire the rights to your bestseller - here's hoping you win the lotto. Because without the rights, no producer will probably touch it.
I hope that helps. And I do wish you luck with your adaptation - let me know when the screenplay is completed. I'd love to read your work!