This book, although historical fiction, is not based on an actual incident. I've only read one other series of real person fiction, and that was closer to fantasy than historical fiction. The author notes the fictional elements in the preface, but the story itself should make it clear to readers that this never happened.
The mystery was well-written, with interesting characters and a consistent tone, but there were some elements of a modern worldview that snuck in. Doyle (admittedly, against the normal mindset) dismisses homosexuality as a live-and-let-live issue, and his religious views are very palatable to modern readers, though the latter may be closer to the trope. Also, some modern terms such as "whirling dervish" appear.
Overall, I think this novel is a decent read, but not historical realistic. 3/5