I had also not heard the podcast but loved reading it. I thought the ending was great. My understanding is:

Old Bret gets off his chest, through telling us this story, that he lied to police about a few but critical details involving his intentions when going to confront Robert. While malicious, I think Bret was acting on good intentions to protect his friends, albeit not with a sound mind. We get presented with the possibility that it was Bret committing murders and/or the attack on Susan/Thom. While I think it wasn't him, I thought it was a great piece of ambiguity and I'm happy to concede that there are things that point towards him being guilty.

Robert is said to be an innocent vicitim of the Trawlers stalking, but I still couldn't understand why he was always going to the house on Benedict canyon? Was it simply because he had access to the house and, if so, wouldn't he have noticed someone using the room and basement? But, assuming I'm just missing something, Old Bret is also getting off his chest that he, through his paranoia and passionate connecting of the dots, was responsible for Roberts eventual death, as Robert was innocent all along.

I really enjoyed how through understanding that Brets imagination and writers intuition had been trusted too much without any real hard evidence, the themes about imagining/daydreaming narratives are clearer. Without a clear line between reality and fiction, Bret looses control and realizes how his made up narrative was deadly. It's sort of a story about story telling. Even the last line mentions dreaming. "...and I would find that record with the platinum-blonde girl on the cover, and turn the volume up, and play it loud, close my eyes and lie back and listen to a song about dreaming."

Thats how I see it. I really enjoyed the book from start to finish. What about you OP?

I felt like it took forever to get to the end and then it was anticlimactic