This is easily the most readable Conan Doyle biography since "Teller of Tales" by Daniel Stashower. Andrew Lycett's slightly earlier biography is, in my opinion, richer on details but is a lot harder to wade through.
The only reason that I have not given five stars is that Miller is guilty of some silly errors. He gets the number of Sherlock Holmes stories wrong and, by omitting certain details, he creates an inaccurate impression of where Conan Doyle was at certain periods of his life.
Miller's book is good but should not be your sole Conan Doyle reference. It will serve you best if you have other books (such as Andrew Lycett's) to plug the gaps that he leaves.