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.
If you've read some Dorothy L. Sayers or Margery Allingham mysteries set in the English countryside, or Ngaio Marsh's English country life mysteries, you will be reminded of some of the scenes with locals. Some of it went over my head, but a lot of it was humorous, mostly in a gentle tweaking way.
On or about page 55, there is a short "Wessex novel" which is a satire on Thomas Hardy's work. If you recall any of them from school, you will enjoy this more. When you find out the main character's name--very funny. And it spoofs Hardy's LONG descriptions as in a scene where the young heroine looks out over the land and sees "a procession making their way over the parched fields (two pages of field description omitted--Editor).'