I am a long-time Holmesian, and I love to see what different authors do with the spin-offs. Some are better than others, of course ... and this is one of the best.
Kareem Abdul-Jabbar, himself a long-time Holmesian, has made Mycroft Holmes his main character in a story about human trafficking during the late 19th Century. With his friend Cyrus Douglas, a free Black gentleman business owner, he goes to Trinidad to investigate disappearances in Douglas' home town ... and becomes embroiled in intrigue at all manner of levels.
Abdul-Jabbar's authorial voice is entertaining, and it's obvious from this work that he takes the Doyle canon very seriously. He gives us a backstory that explains Mycroft's solitary ways in the Doyle canon, and also lets us in on the earliest days of his career in the service of the Crown.
The book is chock-full of entertaining characters, intriguing situations, and great historical detail.