I know this came out last year but my library's copy went walkabout and only recently was returned!
This book's cover is an excellent example of cover design for the following reasons:
1) Appropriate to the fusion of the two genres as this book combines skilfully the Holmes and Cthulhu mythos.
2) Typography An Art Noveau look for the font but note the modifications so that the curls turn from tendrils to tentacles!
3) The three color rule works ! The green contrasts with the white and black
4) Its a wonderful balanced layout of the various elements that makes a clear statement about the book's contents.
Finally its a great read as well thanks to James Lovegrove's knowledge of both genres!