Q. Picked up a paperback (interesting blend of action and horror) at the local dollar store and when I finally got round to reading it, liked it enough to inquire about optioning film rights. They are available.
Tha author's LA agency strongly suggests "partnering" with a producer who will let me write the adaptation. They also would be receptive to an offer by me.
I don't have a strong relationship with a film maker to trust not getting cut out of the picture. That leaves the scenario of making an attractive offer, writing the thing, then doing The Shopping (without an agent, of course). Sheeesh!
Here's the thing. A bestseller is valuable to a producer or a studio. It is a bankable element. There is a proven audience for this world and this story. Most books you find at the dollar store are proven to have a small audience. So basing your screenplay on one may not be helpful.
On the other hand, how faithful an adaptation do you plan to make? You can't copyright an idea. You can only copyright the expression of an idea. E.g. no one can copyright "girl romanced by vampire"; they can only copyright a girl named Bella being romanced by a creepy stalkery sparkly vampire and a hot werewolfey dude on the West Coast, etc. etc.
Novels don't usually adapt well to movies, because the plot of a movie is basically a short story.
(Some novelists write in a very cinematic easy-to-adapt style. There are reasons every John Grisham novel gets an adaptation, and being best sellers is only one of them. They generally have only one or two points of view, and time flows at a regular pace. There's no, "Over the course of the next few years, Johnny came to understand...." Everything that happens in them is a scene involving at least one of a core cast of a few people talking, fighting or going somewhere in a hurry. There are few flashbacks. You always know exactly what is happening and who it's happening to. The characters have very little inner life unless it's also expressed in dialog. Etc.)
So the best adaptation is often an unfaithful one, or rather, one that is faithful to the theme and the spirit of the novel, not the details of the plot. An introverted character may need to become talkative. A long thought process will need to become a verbal argument. You will almost certainly merge characters and cut others.
It may be that your unfaithful adaptation winds up so far from the novel that you really only have to change the names of the characters, and you no longer need to option the novel.
Now, I'm not a lawyer, and no lawyer is likely to tell you exactly where that line is. But consider that Fifty Shades of Grey started out as Twilight fanfic. And consider that there are a bazillion books and a bazillion scripts and really not that many plots.
This approach will also free you to write a better screenplay. You can usually tell a movie adapted from a book because there are scenes that are cool that don't really forward the story: that's the screenwriter trying to keep a scene from the book that really doesn't fit in the movie.
So what I would do in your case is read the book and then put it away somewhere. Do not read it again. Write a script based on it. And by "write a script," I mean, as always, tell it as a story, orally, over and over to anyone who will listen, until you can tell the entire story off the top of your head because it flows so naturally. You will wind up adding scenes. You will forget a lot of scenes; oh well, they were forgettable.
After you've finished writing the script, reread the book and see if you have stolen anything copyrightable. My guess is you will not have.
You have now saved a few thousand bucks, and you can't get removed from the project.
Not everyone will agree with this advice, and the line between copyrightable and not copyrightable is not bright and clear. But the times I've optioned material it's generally been a pain in the ass that I optioned it, especially because the script came out so differently than the source material. So there you go.
Note also that this process is not how producers generally approach adaptation. That's because (a) they have money (b) scripts cost more money to them than options (c) they can take a book and an option to someone else to give them money. Hence the above is not how, say, Alfred Hitchcock's The Birds
gets made. But if you're a screenwriter without a track record, then optioning a book may not be the best way to go.