"The Misfortunes of Mr. Teal" (aka "The Saint in London") is one my favourites of the books of (usually three) Saint "novelettes", the other being The Holy Terror.
Chief among its pleasures is the finale to the battle of wits between Simon and Rayt Marius, the unscrupulous arms-merchant of The Last Hero and Knight Templar. Marius, learning that he has only a few months to live, has devised a scheme, worthy of his evil genius, for revenging himself on the Saint: he has written his memoirs, detailing all the criminal (indeed, treasonous) transactions he has had with members of British government and industry, and sent the resulting opus to Simon.
"At the same time as this book is sent to you, there will be sent, to the gentlemen most conspicuously mentioned in these notes, letters which will inform them into whose hands the book has fallen. After reading it yourself, you will see that this cannot fail to cause them great perturbation.
"Nevertheless, while it would be simple for you to allay their alarm and assure your own safety from molestation, I cannot foresee that a man such as I recall you to be would so tamely surrender such a unique opportunity to apply moral pressure towards the righting of what you consider to be wrongs.
"I therefore hope to leave behind me the makings of a most diverting contest [...] And you will understand, I am sure, my dear Mr. Templar, that I can hardly be blamed for sincerely trusting that these gentlemen, or their agents, will succeed where I have failed."
This story is also notable for introducing the Runyonesque American gangster Hoppy Uniatz, veteran of Prohibition who drinks whisky like lemonade, the Saint's companion in so many later adventures.
That Charteris manages to make Hoppy simultaneously menacing, comical and believable (and even at times sympathetic!) is an amazing feat, but he pulls it off — in part because of a firm grasp of Transatlantic mores and idiom, much better than (for example) Ian Fleming's.
The other stories are just as skilled; there is little point in detailing the plots, and again the familiar friends — Pat, Orace, Claud Eustace — are here.
One of the best books in the series.
P.S. For a list of — and discussion of — all Charteris's Saint books, see my So You'd Like To... Guide.