I freaking love this series. I only wish there was longer between the books to give a sense of... I dunno, tension, suspense, anticipation? It doesn't seem right publishing them one after another over just a few months.
But on to the book itself.
The start of this book is so incredibly different from the start of the first one. And the ending? GIVE ME THE NEXT ONE NOW! You're dragged in by Steampunk pirates and a story that flows different from the first in the series - the writer has matured. The pace is different too. Where A Study In Silks was a little slow to start, this one is all action.
This series is really into the whole building of the world thing that can make or break a book. And it does it extraordinarily well. You learn more and more with each page but there are never any info-dumps or boring bits. This Steampunk world is so complex and so real.
The ending was just... Wow. It was so shocking and totally unexpected. I actually gasped out loud.