A comment on the back cover calls it "The Classic Werewolf novel".
The book starts well with a great opening sequence as a reporter, Will Barbee, and a strange red-haired woman, April Bell, await the return of 4 scientists from some strange archaeological dig in Mongolia. The head scientist announces he has a message that must be heard before evil forces have time to attack him, and just after he reveals the threat of a Black Messiah "The Child of the Night" he dies on the spot, seemingly from natural causes.
Barbee is drawn deeper into the tale as the scientists, who once were friends of his, also start dying in odd circumstances. On all those occasions he dreams that he is involved, that April is drawing him into an evil conspiracy; or could his odd dreams be more than that? And what is in that odd box they brought back?
Originally written in 1940 several ideas that were fashionable at the time get an airing that may not be as relevant to modern day readers. The work of JB Rhine and his experiments in parapsychology and telekinesis and Heisenberg's uncertainty principle are drawn on which leans it more toward a science fiction approach than a straight horror tale.
As a whole the book works well as it builds up the suspense, the identity of the Child of the Night can be guessed at as the story develops but that does not lessen the overall impact of the story.
Is it the classic werewolf novel? Probably not, there are too many shape-shifting deviations from the werewolf theme. My vote would be for "The Werewolf of Paris" by Guy Endore (written in 1933) which is as close as the werewolf genre comes to having a classic novel as Dracula is for vampires.
Is it a good novel? It certainly is, and is well worth reading.