This was a pretty good read. I downloaded it to read while commuting and after a couple of trips, wound up finishing it at home that evening instead of TV.
Kind of like reading the story inside a video game, once you really get into the story.
I believe this was written for the author's son's new publishing firm back in the early 1900's. There is some humor in it, the mystery is indeed rather Hardy-Boys-like; I think the atmosphere is the big thing. Rinehart was later satirized by Ogden Nash as the writer of "Had I but known" stories: "I wouldn't have bought it had I but known/It was filled with "had I but knowns."
Skip the librivox audiobook; at least one of the narrators, along about the 16th and some of the following chapters, is very hard to follow. Her first language does not uses articles as English does; for example, she says, "Chapter Sixteen, Circular Staircase," and otherwise leaves out all the "the's." Her words are audible, her tone agreeable, but many many words are just not intelligible--including part of her name.
Early days in Norbert Davis' writing career, I think, and written for the "pulps." Characters are flat, not real people at all. Plot was kind of rickety but some of the dialogue was fun. Davis developed Doan's character more as years went by (about 5 stories by Davis on manybooks.net as I write this, last one copyrighted 1943). Other characters also became a little more rounded, and his female characters are very well done. Doan will be a stronger, more individual character in the later stories, and Carstairs gets MUCH more room in the later stories. Here, he's mostly a gimmick.
Read about 3/4 so far, enjoyable as description of aristocratic/diplomatic life in early 20th century. Not much detail, a certain level of understanding of "how things are done" is presumed.