It was a reasonably fun episode, a good try to emulate the '40s detective milieu, but the cinematography was too modern. I was spoiled long ago by Moonlighting
's film-noir episode "The Dream Sequence Always Rings Twice"; that show's cinematographer, Star Trek
's Jerry Finnerman, was trained by the generation of cinematographers who'd cut their teeth in the film noir era, and so he was able to replicate their approach to a tee (plus he shot in black and white). So this felt like a pale imitation in comparison. The music also felt a little inauthentic, like it was trying too hard but not really capturing what it was imitating.
I also have a hard time buying that the diary was actually written in the style of a hard-boiled detective novel. It would be one thing if they'd established that Joe was an aspiring author who'd chosen to dramatize the case in that style -- or if they'd had the diary itself be fairly prosaic and made it Castle's imagination that interpolated the Chandleresque elements. Then there's the coincidence that the case just happened to involve people who were good fits for each of the show's main characters, not just in role but in appearance, age, and ethnicity. The whole thing was just too contrived.
(Spoiler paragraph) And as soon as they revealed that the bodies in the car had been burned, I knew that Joe and Vera had faked their deaths. It was obvious for two reasons: the in-story reason, that there's no point in burning two people who've already been shot to death unless it's to conceal their identities (and it was weak writing to have none of the detectives realize that), and the metatextual reason, that after getting us to identify Joe & Vera with Castle & Beckett, it was unlikely that the writers would give them a tragic ending. Although I only realized that the elderly couple were Joe & Vera around the same time that Castle was putting it together.