I absolutely loved this book!! Falkner does well throughout the book not to get bogged down in unnecessary details. Every page is relevant and "meaty". I wish he'd been a little more descriptive with the characters' physical traits, but for the most part this is the best "let's get to the heart of the matter and forget the inconsequential details" book I can remember reading. And the story is superior.
It's full of weighty issues like: greed, death, sorrow, joy, repentance, love, compassion, redemption, betrayal, youth, wisdom, fate, and forgiveness. Even with such heavy topics, it's written in a way that young and old readers will get the maximum out of it. I'm 46 and didn't feel like I was reading a kid's book at all, though I think middle schoolers & high schoolers would enjoy it and benefit from it too...maybe even more than mature adults.
Chris de Burgh (who sang "Lady In Read" back in the 1980s) has a CD called "Moonfleet & Other Stories" in which he does an excellent job through songs & periodic narrations to accurately & concisely tell the story of Moonfleet. It will spoil the ending for you, so don't listen to the songs if you don't want to know the ending.
Lastly, it's just an excellent story. It flows from chapter to chapter as though John Trenchard (the main character) was telling you the story in person. It's easy to see why this is an English classic.