Slow Decay takes everything that works from the TV Show and expands on it.

A strange, esoteric alien threat, but with a real human villain. Great character conflict and general human drama, including Gwen and Rhys having some really great relationship drama that doesn't involve anyone cheating on each other, and some great characterisation for the rest of the team, especially Owen.

Also just the descriptions are incredible. This book made me viscerally uncomfortable at points, both with the graphic descriptions of blood and gore, describing just enough to leave the worst to the imagination, and with the inner voice that you get at points. One moment in particular comes to mind where Rhys and Gwen are arguing, their emotions boosted by an alien device, but the emotions and thoughts still being their own (and explicitly called out as such). That moment hit me especially hard, and it's extremely well written.

The plotting also feels really well paced, everything coming together really nicely with solid explanations and payoffs to everything that's set up.

I could see this being an episode of the show easily, it just slots right in perfectly as well as telling a really compelling story, and honestly, what more could you want?