Leroux stated in this book that his purpose was to creat something totally different from traditional detectives such as Poe's Dupin and Doyle's Holmes, his mission was very well accomplished. Not only the mystery is out of the traditional line, but also the solution.
Strictly speaking, Poe and Doyle never wrote locked-room, at least they did not intend to write. In this novel, we find totally fresh mysteries: murderer escaped from locked door and barred window; murderer disappeared when chased by people from several directions in a gallery. If a reader is very familiar with Carr's mysteries, the above poses no difficulties. However, DO remember, Leroux is the creator while Carr is just a good (or not so good) imitator, only by this way can one fully and truely appreciate Leroux's originality and great contribution.
Despite the plot, the narrative is Sherlockian style, which can keep readers thinking, and is much prefered to Carr's leaping style.
There is inevitably some weakness in the novel. The major one lies in the research paper, whose disappearance and return are not at all well explained. Since the plot can well stand alone without the paper, its presence is really excessive. Besides that, what I hate is the detective's from-nowhere remarks, which are purposely employed to show the detective's "superhuman" ability by making readers totally dumbfounded, and which then disappointly turn out to be something from his eavesdropping. And I hate the everything-is-possible-with-Mr.-super-villain explanation, why not just let the murderer be an alien, with whom everything is more possible?
In conclusion, this novel is worth reading, not only in its historical sense, but also as a detective story. One will not be disappointed if one can tolerate Carr's novel.