Shiftling is an excellent example of how good the novella format can be. You take a single idea, a small cast of characters and really dig into the story without padding it into a novel. For darker fiction, I think the shorter format is perfect. It can hold the suspense more intensely over a shorter period of time compared to novels, which often dilute that intensity. That's not to say there aren't brilliant horror/dark novels, just that when you have a single, strong idea, the novella form makes it shine.
This is a story split across three parts of the same timeline. What could have been a fairly common trope and structure, Savile manages to weave the plots expertly, ratcheting the tensions with each short chapter, shining a light into the darkness of the protagonist's story one scene at a time, until it builds perfectly to the final reveal where all three plots merge into a fantastic climax.
I think readers aged 30+ will likely get the most from this book as Savile does a wonderful job of conjuring the 80s, citing the bands and records of the time. I related to a lot of this story, both in terms of the kids' angst, the camping out in hideouts, the mysteries of waste grounds (I'm sure we all in the UK grew up knowing about a bit of wasteland or dodgy woodland somewhere), and the curious, adventurous nature of being a young teen—and all the baggage and awkwardness that age can bring.
Throw in a creepy house, with a creepy owner (we've all known of those as a kid, surely?), and a legacy that continues to haunt the protagonist into middle-age, Shiftling is a solid, occasionally whimsical, creepy tale that won't be soon forgotten.