Mrs. de Winter is the novel's main protagonist, and her monster, in terms of a Booker analysis, is Rebecca. She's only dimly aware of Rebecca as a threat when she meets and falls in love with Maxim de Winter, a brooding and handsome man of wealth and privilege.
All of Mrs. de Winter's wildest dreams seem to be coming true. She's married to Maxim and coming to live at Manderley, the kind of place dreams are made of. At the same time, she's moving closer to the monster (Rebecca) who seems intimately connected with everything important at Manderley.
Marriage to Maxim and life at Manderley isn't all it's cracked up to be. Mrs. de Winter fears that Maxim still loves Rebecca. She's also afraid that everybody thinks she's a poor substitute for Rebecca. These insecurities all increase to manic levels, driving her to, well, the next stage.
Mrs. de Winter is conned by Mrs. Danvers into dressing up like Rebecca once did for the great costume ball. Then, when she thinks her mistake has ruined her marriage, Mrs. Danvers almost convinces her to jump out a window. Nightmare, indeed. Doesn't get much worse than that.
Yeah, Rebecca is a monster who is already dead. But the discovery of her body forces Maxim to confess that he murdered her. Yikes. Since he killed her out of hate and not jealous love, Mrs. de Winter can feel secure that Maxim loves her. (Sounds a little sick, doesn't it?) After that, Rebecca's death is half-heartedly investigated and eventually ruled a suicide.
But wait a second: In a way, Rebecca is actually the antithesis of the Booker's "Overcoming the Monster" plot, since the protagonist's monster is actually a terminally-ill murder victim. The real monster, perhaps, is her murderer and the accomplices in the cover-up, and he's not punished or overcome in any traditional sense. The real monsters in Rebecca are inside the characters, and these monsters are far from overcome when all is said and done.