Most of the time, a prequel or sequel isn't as good as the original. When it takes its departure from a classic, beloved by generations of teenage boys (or adults reliving their youth), a prequel is an even higher hill to climb.
However, I enjoyed "Flint and Silver" from beginning to end! Author Drake has gone to a lot of trouble to study the original "Treasure Island" and bring back familiar characters like Long John Silver, Cap'n Flint (both the man and his parrot), Billy Bones, Ben Gunn, Blind Pew, and Israel Hands. He has made them consistent with their roles in the main event to an impressive extent. The author's Afterword (worth reading) explains why he decided to modify Israel Hands and essentially create a new character, Selena, to inject adult romance into the situation, since writing in his mature age, he preferred not to stick with the youthful point of view of the original.
I haven't reread "Treasure Island" in more than 20 years, and honestly didn't recall that Mr. Stevenson had given us a one sentence reference to the black woman who was Silver's housekeeper, or something. However, in one of the many movie versions of "Treasure Island", she was his wife, and author Drake either picked up on this or decided to introduce a romance on his own. I don't think that's objectionable; it adds to dramatic tension as the two main characters of the title become rivals for her favor.
Other than those differences, this book is an action-packed pirate adventure, with plot twists happening on practically every page. It doesn't matter that we know it's going to end with the treasure buried on the island, and Silver back in England; it held my attention and kept me glued to the text for hours. I had my doubts if Capt. Flint would really have sailed into the anchorage without sounding it first just on the strength of Portuguese sailing directions, the sequence that strands HMS Elizabeth near where Jim Hawkins beaches Hispaniola in the original book. Other than that it was totally authentic.
It is bloody! But pirates were like that. Making Capt. Flint into a cold-blooded murderer is consistent with what's in the original, even though we don't get to see him alive in that book.
An element of speculation is the pirates' motivation for burying their treasure. Now that I've read the afterword I recognize that there's a certain logical inconsistency to this, but I like the author's conclusions. When you have tons of treasure and want to keep it all, you might not be completely rational. Tales of pirates burying their treasure are a staple of New World myth, so author Drake can be excused for taking it at face value even though he's writing from the UK.
It's verry much worth your time.