The author is deceased. Keep in mind that anything I say here should be checked with your lawyer. I am not a lawyer, nor do I play one on TV but here's what I think, anyway. It's good that you realize you should not do any work on an adaptation without the rights clearly assigned to you. Other wise, it's wasted work.
Most publishing contracts let the publisher share in the movie rights money.
In addition, the publisher may have some say regarding the decisions about selling the movie rights and for how much. That relationship between publisher and the original writer usually remains even though the publisher lets the book go out of print or the author dies. A contract could be in force until the work loses all value and goes into the public domain. If you contact the publisher about the where-abouts of the writer's heirs they may be forthcoming and put you in touch with them. More likely, if they own a percentage of the movie rights, they will want to negotiate the rights before bringing in the heirs feeling that the heirs will not demand enough money for the movie rights.