How did I feel about this overall? I do have some mixed feelings. First, I started it on Aug 31 and finished today Sep 29, so it was about a month of reading gradually. I do think if it was more interesting or written at a better pace, I'd've gotten through it faster. I didn't think everything was of high relevance.
The biggest issue is the fear of the "fire" consuming them all. It wasn't too bad in the beginning of the book, but toward the end, it became such an integral focal point that it was rather annoying as it was technically totally disconnected to the mystery. (Indeed, the solution of Ms. Xavier being the murderer of her husband doesn't use the fire at all, and it could've been cut.) I never felt any true danger from it, and I was always expecting rain to eventually come, even after the rescue/messenger plane dropped a message saying no planes could land amidst the blaze.
I'm not sure how the author could've written the fire to be more interesting or tense...but honestly, I wish it were just removed. It isolating the murderer is fine, but it didn't need to be brought up so much more. Similarly, there was a bit too much focus on the medical aspect of taking care of the wounded. Ex when Ms. Xavier is hysterical and watched over or when the brother of the murdered person is accused and panicks, flees, and is shot and then patched up...there's a lot of description of the scene of what happens and such, and I think it's not actually of high relevance. Basically, the book had a lot of moments where it could've abridged the details while still getting the point across.
Then there's the actual logic of the case itself...I do not love the usage of the "Carreau" last name meaning "Diamond" hence a relation to the family/twins, even if it was a fake clue by the murderer. I prefer clues that are more reasonable for a reader to catch onto as well, though I can forgive this in that this was before the final reveal and that it was ultimately explained as a fake planted clue. (And the explanation made sense too - though I already did think it was a bit of a stretch for a dying person to grab a card like that with the murderer not bothering to check at all before leaving.)
Aside from this, I also didn't like the supposed deduction that left handed or right handed people ripped a certain way and kept cards in a certain way. Yes, it is like that in the majority of cases, but you cannot rely on that in all cases. Some people use the opposite hand sometimes on a whim, and I dislike this being the fact to "prove" that the leftover cards were fakes. But I suppose I should accept this as it was indeed explained far before the actual solution.
Now the actual solution itself...it was clever, and I did not get it, and I acknowledge it was well played. The twist that the murderer framed someone else, then someone came along and unknowingly framed the correct murderer is a nice twist, and the book was split into a nice 4 ish parts with even the correct murderer confessing about halfway through but then backtracked to fully solve the case. I can even accept the innocent brother fleeing in panic because he was truly the one to plant the fake evidence (though again, I think the idea of "X used L/R hand so clue should be left in L/R hand" to be a stretch still).
The number of twists leading to the final solution was nice and well done. I also did like Ellery and his father as characters, and the rest of the cast was good too. For example, when the fire was aobut to consume them all, I like that Ann Forest ripped off her whole dress in the darkness, so they could have something to stuff the door with - it makes sense in terms of desperation, and I thought it added a nice bit of flair/characterization.
Similarly, I like the idea that Ms. Xavier stole rings as a kleptomaniac, and this was shown with Ellery's father's ring being stolen (but not his key to the card safe, which proved the cards held a fake clue, very rightly so; though, I had taken this to mean the murderer was in a rush to leave quickly and didn't want to search deeper vs just picking a ring off the finger, as Ellery was due to come later that night. I think my interpretation is a believable one, but I think the author's explanation makes sense too and ties together perhaps a bit better and cleanly enough).
Now, to return to my original review - did I enjoy it as a whole? I'd say the beginning was a bit flat, focused on introductions. I think when the first murder occurred...it honestly wasn't THAT interesting. I especially didn't care for Ellery ripping cards to test the manner cards are ripped, as again, I dislike his insinuation of hand being so closely tied to the manner of ripping. Then the middle did get a smidge better with more tenseness of who the murder is and the expectation that the wounded brother would stir and that there'd be an attempt on his life. That certainly did get my attention.
But...the main saving grace of the novel was the climax - everyone wounded and tired in the basement waiting death by fire...only for Ellery to reveal the true culprit to be the original person his father first arrested (and also the first person to honestly confess). I did like that twist, and his explanation tied in perfectly fine. I'd say this saved the novel from being a "bad" novel for me, as it was previously a 1/5 at that point.
Now, as to whether I could rate it higher - no, I cannot, or at least, I'd say I didn't enjoy it that much at the moment and still don't think it's that enjoyable a book. I still didn't enjoy it for the majority of it, didn't agree with all the logic/didn't like the way all the clues were done and used (ex the French last name that most readers wouldn't know, the manner of ripping being hard to believe, etc), and disliked some random things like Bones picking the gasoline off the cars to prevent Smith from leaving since he "disliked" the guy's face, which is never addressed again...these and some other small things like the excess focus on the raging fire dampened my reading enjoyment too much to say it was a good read.
It could've been written a bit tidier/more succinctly, but I also admit it was very cleverly plotted as a mystery (I guessed Ms. Wheary at the end, as she was the one to call attention to the safe again, but I was obviously incorrect). If I had solved it myself, honestly, I might've liked it more, but the above minor issues and the excess detail at times dampened the experience enough that it wasn't that good to me. Not bad at least due to the cleverness but also not that good due to the issues above.
I may try out other Ellery Queen books though. This at least made me respect the author, as I felt the mystery was more cleverly and fairly plotted than Agatha Christie at least, and I heard the early [Country] [Noun] mysteries in the series are all pretty fair and good. So, I may try them and think the author duo did a good job, even if I didn't love it all.
EDIT: Small nitpick but the edition I read didn't have "Challenge to the Reader", and I was quite looking forward to it. I read other editions have this though, so I suspect this is an issue due to the publisher of my edition, "American Mystery Classics" - perhaps I'll track down a different publisher for future Queen books I read.