This book definitely suffers from being an older book -- lots of derogatory reference that (hopefully) wouldn't be included in books today. The overall mystery was interesting but there were a lot, a LOT, of red herrings that resulted in some last act twists and turns.
Anthony Cade is leading tours in South America when one of his friends comes up with an assignment. If Anthony delivers a manuscript to a London publisher by the 13th, his friend would get 1,000 pounds and he'll give Anthony 250. Anthony is ready for an adventure so he takes it on. He arrives safely in London but his first night there, the waiter who delivered his dinner breaks into his room and rifles through Anthony's suitcase. While the manuscript is safe, the packet of letters that someone had been using for blackmail is gone.
A dead body that shows up in the lounge of the person who was ostensibly being blackmailed, another a shooting occurs at a house party just when Anthony is sneaking around, a country's throne is at risk, and Superintendent Battle is called into the scene.
Three stars
This book came out in 1925
Followed by Seven Dials
Borrowed as ebook from Libby
Opinions are my own