Not much to say about Agents of Light and Darkness. It was short but fun; Green is good at juggling a humorous tone with some darker undertones.
Casanova's Chinese Restaurant is the fifth book in A Dance to the Music of Time. Although you can read the whole work as one massive volume, this novel shows that Powell paid close attention to the structure of each individual book. The novel opens with Jenkins reminiscing about the titular restaurant, reflecting that there's now a crater where the restaurant used to be. The crater is the result of bomb dropped during the Blitz, and is a sign that events leading up to WWII are going to become more prominent now. Indeed, this is the first novel where the outside world really intrudes, however lightly. One character goes off to help out in the Spanish Civil War, there's more talk about the Nazis, and so on.
Casanova's also underpins the structure by book-ending the novel. We're introduced to a couple of major figures in this novel at a meal there near the beginning, and there's a reflection back to that meal near the end. In addition, the restaurant serves as a bit of pivot point between past and present. We have Jenkins's current present, in which the restaurant is now a crater. We have the "present" that most of the novel is set in. But we also have a past before that, in which Deacon (who passed in away in vol. 2, if I recall correctly) shares a meal with Jenkins and others at the restaurant.
There are also a few pairings which undergird the structure of the novel. Most obvious are the two musicians, Morland and McClintock. But we also have two miscarriages at the start of the novel, linking Morland's and Jenkins's marriages. McQuiggen and Members are still paired, as in previous volumes.
The one complaint I have is that there's a huge hole in the novel where Isabelle should be. Powell tries to get around this by having Nick tell us that no-one can write objectively about his own marriage, but that feels sloppy to me. Nobody can really write objectively about any marriage (as Nick also tells us), but other peoples' marriages fill the pages of these books.