Oh, it absolutely does. The connection's more of a shift in gears, honestly. 'City of Fallen Angels' picks up a couple months after the big showdown in 'City of Glass', and it's like... okay, we saved the world, now what? You get to see the emotional fallout, which the earlier books kinda glossed over. Simon's whole vampire-in-the-sunlight thing gets a major plotline, and Jace starts having these nightmares that feel disconnected at first but seriously tie back to the broader demonic politics of the Shadowhunter world. It's the book where Valentine's gone, so new villains start creeping out of the woodwork, setting up the whole Lilith and Sebastian arc that dominates the next two books.
Some people say it feels like a standalone sequel, but that's missing the point. The relationships established here—like the tension between Jace and Clary, or Simon and Isabelle getting closer—are crucial for understanding their dynamics later. Also, the introduction of the Mark of Cain on Simon? That doesn't just go away; it becomes a key factor in the final battles. So yeah, it's not a detour. It's the bridge from the first trilogy's clean ending to the much messier, more personal conflicts that follow.