I'm a longtime fan of British mysteries, particularly the traditional British detective type of mystery. The series that begins with this book has turned me into a Sherlock Holmes fan. I know it's a little backwards to become a fan of the original by first becoming a fan of revisionist fiction (or, when we're at home with ourselves, glorified and published fan fiction), but there you have it. That's how I got into it.
More spoilerish type review ahead ...
It's a good exercise in the whole suspension of disbelief idea at first: that Sherlock Holmes would actually take a woman as an apprentice, that he would eventually fall in love with her, and that it is reciprocated by this woman who is about 40 years his junior. I grant that there are relationships between older men and younger women that are intellectually-based. I'm one of those who's dated and been much more comfortable with men ten to sixteen years my superior in age.
But.
Anyway, the book won me over. I put aside all of that disbelief because the writing was good. The historical details were intriguing, the pace was great, and later books in the series are richly detailed. So. Yay. I'm sucked in, and I read every published book in this series within two months of reading this first one. That ought to say something.