This is a great mystery, told in the same sparse but eloquent style used with the Holmes stories. I could, and did as I was reading, nitpick about a few things, which is why I went with 4 stars. Don't ask me what they were though; I just finished it, and yet can't put my finger on what I found lacking.
But the book represents something far more fascinating than another solid Conan Doyle mystery; The Mystery of Cloomber is an excellent representation of the two sides of Conan Doyle: the empiricist and the spiritualist. Originally published in 1888, this story shows that ACD's embracement of the occult was a gradual process that spanned decades. WWI might have been the tipping point, but as this mystery attests, the foundations were firmly in place early in his career.
The book is also interesting for the setting: if you liked the bleak moors of England in The Hound of the Baskervilles, you'll enjoy seeing Doyle's earlier use of the moors (this time Scottish) in The Mystery of Cloomber. Like Hound of the Baskervilles, the moors play a big part in the plot, though their mysterious and heavy atmosphere are used to less effect.
i don't want to say more about the plot for fear of spoiling it, but I'll leave off with this: this mystery might not have a satisfactory ending for some readers. There's a resolution, but, well...