Dr. Jekyll and Mr. Holmes by Loren D. Estleman

Accompanied by Dr. Watson, master sleuth Sherlock Holmes has already encountered the evil young hedonist Edward Hyde, and knew he was strangely connected with Henry Jekyll, the wealthy, respectable London doctor.
It was not until the Queen herself requested it, however, that Holmes was officially on the case of the savage murder of Sir Danvers Carew-the blackest mystery of his career! Although Robert Louis Stevenson published his tale of Jekyll and Hyde as fiction, the hideous facts were true, insofar as Stevenson knew them.
Here, then, is the entire firsthand account of that devilish crime as recorded by Dr. Watson, with an explanation of why Holmes's personal involvement had to be kept secret-until now...