Had a quick look just now. Clean syntax for the implementation, but routing and parameters are relying on magic # annotations. Compare with e.g. https://doc.akka.io/docs/akka-http/current/routing-dsl/index... where you can do the routing in plain old code (also async is already there).
(I hate akka and actors, but as a user of akka-http you don't have to touch them.)
Actually, as someone almost finished with the book, that brings up an interesting point. What controls # annotations and how do you hook into them (which also answers the question of how magic they are)?
If it's fairly standard and obvious how to look up, I think I might not classify them as much "magic", but truthfully, I have no idea at all. Maybe I'm just not remembering that part of the book (I did take a break for a month), or maybe I didn't make a connection that was implied?