I read this as a buddy read, but went on vacation at about the halfway point. My hotel wifi was pretty worthless, so I wasn't able to login while I was Disneylanding - and Disneyland isn't really a place for internet activity, in any case, because by the time I'd get back to my room, I was so exhausted that I sort of just collapsed into bed!
Anyway, I quite liked this fun mystery. There is a lot going on, and I found the main character to be a hoot. She has very little instinct for self-preservation, and is a bit bumbling, but is also funny and brave and loyal. I did figure out some of the elements and had a pretty good idea of the solution before the reveal, but the complicated plotting was well-done.
Mary Roberts Rinehart is sometimes referred to as an American Agatha Christie, but this book didn't read like Agatha to me, nor did the other book by her that I've read, The Window at the White Cat. Something about her writing style is peculiarly American, and there are hard-boiled qualities to it - she's not noir, but she is also not cozy.
A lot of Rinehart's books are in the public domain. My kindle version of The Circular Staircase only set me back $.99 - for readers who enjoy vintage mysteries, I'd recommend giving her a try.