You are always going to get filler episodes somewhere along the way and this felt like one.
No real action, no real developments just a progression towards the finale. Is Bosch closing in on Waits? Well, maybe if he gets something out of the files from the boys home. Has Bosch ruined his relationship with Brasher? More than likely. Did Irving look cosy in his Christmas sweater? Definitely!
With the fortuitous finding of the photos in the art book it is possible that Waits is trying to recreate the catacombs, which would link into the tunnelling that he was up to in an earlier episode, but he doesn’t keep his victims so where is he going to get his supply of bones?
The killing of his mother has been brewing, you could see him getting more and more frustrated by being beholden to her and this is the easiest way out for him. Now he can truly be himself and not be taking risks doing day to day stuff for his mother.