It's a very good book, but it is far from being one of Doyle's best works, let alone the best crime thrillers or detective novels in the world. There are some points to consider about this book.
Pros:
•It is a very well-placed narrative and one guarantee that can be ensured is it is going to keep the reader hooked till the very end and one would never get bored.
•The villain is a very shady, cunning and deceptive character which shows a couple very planned-out and gruesome murders.
•It provides an angle to some of the future stories of Holmes and Watson.
Cons:
•Though it starts out well, the middle is filled with reports and letters from Watson which makes the book incredibly slow-paced. The pace is picked up towards the prelude to the climax.
•It shows a lot of clever deductions of Holmes but very few are actually connected to the case. The unfolding of the case kind of just... happens.
•They start with a lot of false threads in the beginning but soon near-about everything seems to go the detective's way. Also, a lot of questions aren't really answered satisfactorily by the end.