Portuguese pulp fiction, sadly, is often just a well-known American or English writer in translation. And a careful skim of O Santo e o mistério de Lisboa turned up no conclusive evidence that Leslie Charteris ever set foot in the city. But check out the prefatory “Warning to the Reader”:
For your own interest, prized Reader, check to see that this book still has the white wax tab that seals some of its pages: if so, open it, please, as you would an uncut book, that is, with a knife, or even a simple card, and that way you will avoid tearing the pages. If you find the book unsealed, get rid of it, because this indicates that it has already been read. Protect your health by not handling used books.