It's a Mad Mad World

I got a mail from someone that had read my review for “The Elementary Sherlock Holmes", and he asked me for a favorite title if you wanted to know more about Sherlock Holmes (Since I found that one a bit lacking if you want to know more) and I realized that I have never read a really good book "about" Sherlock Holmes. Sure I have read fictional books, the real stories and seen a lot of movies and tv-series. But a real honest to God book about Sherlock Holmes have I never read (as far as I know).

So my fellow Sherlockians. What would you recommend (both to me and to the guy that wrote the mail)?