I made it through Chapter 17 last night. I can't beat Themis Athena's excellent breakdown of the cast and plot thus far, so I'll just add this:
Is it just me, or do these Golden Age mysteries always make London feel like the smallest city ever? I swear. If you need to run into someone you haven't seen in ages, go to London. If you need to run into someone you only saw once from behind, go to London. If you need to run into someone you've never met who is crucial to the plot, go to London.
London: The Biggest Smallest Foggiest City on the Murder Mystery Map