Is the ending of Mr. Brooks (2007) an open one?

From what I can gather, you are both right and wrong! Right in feeling that the ending was open ended, but wrong in suspecting that the whole thing might have been a dream - at least, I can find no evidence to support this theory, neither in the movie itself nor from the Writer/Director (Bruce A Evans). I find it hard to believe he would not mention something as important as this aspect of the movie if it really was meant to be suggested that the whole thing was a dream.

Mr Evans) has stated in an interview on Movieweb.com that the ending was left with a few unanswered threads:

When we wrote it obviously the ending is open ended and Earl is in great pain because of his daughter. There is an open-ended... neither Earl, the daughter or Atwood has come to a conclusion.

This is due to the initial desire to make a trilogy out of the story, as further mentioned in another interview on bloody-disgusting.com

“One of the initial ideas that we weren’t able to fully incorporate into the first film [was that] Mr. Brooks was fascinated by [Detective Atwood],” Evans says. Though Demi Moore was cast as Atwood in the final film, her part was initially written for someone a bit younger, allowing Brooks to begin feeling oddly paternal toward her. “He was impressed with her. It wasn’t a love story. He saw her as the daughter that he’d always wanted. He’s tortured by his relationship with his own daughter [who ultimately reveals herself to be as murderous as her father], whereas [Atwood] represents who he had always wanted as a daughter. And that engagement would have continued on with the second film.”

The bulk of the third film would have found Brooks’ daughter Jane turning against her father and framing him for murder, setting up a battle of wills between the two as the younger Brooks tries to “get back at Daddy”.