The Monsters Inside is very notable for being the first tie-in novel in the new series to be referenced directly in an episode. That basically makes it indisputably canon, and it’s a shame that a rather “meh” novel like this would get that honor. Stephen Cole is a very good writer, who has contributed great things to the Doctor Who mythos, but this isn’t one of them.
As if the “Aliens of London/World War Three” two-parter in the series itself wasn’t enough to establish that the Raxacoricofallapatorians are a terrible alien species, The Monsters Inside brings them back pretty quickly and drives the point home. They’re awful. Whether it’s the Slitheen or the “Blathereen,” as the primary problem family is called here, they are just useless and not interesting at all.
Luckily, Cole at the very least gets the characterization right. Unfortunately, this is not used to its full potential because Rose and the Doctor spend the majority of the storyline apart. This wouldn’t be so bad if the storyline were better, but it’s really not particularly interesting here. The Doctor and Rose accidentally land on a planet that is being used as an experimental prison colony, so they are forcibly imprisoned and spend the entirety of the novel trying to find their way out.
I love a good dystopian story, but The Monsters Inside never quite achieves the mood it should. The Doctor and Rose being forced into a space prison should feel a lot heavier than this story does, and that stops it from being as compelling as it could have been. There’s nothing wrong with the prose, supporting cast, and the rest of it, but the mood is all wrong and the Blathereen are just another family in a species of terrible aliens.