Between a 3 and a 4 for me - a much better book than Mindwarp (and I don't have Revelation of the Daleks currently), and so a much better and interesting read.
Kicks off the Trial of a Time Lord arc, and here it puts an interesting overlay over the events, as there is extra mystery provided here, which we don't get all the answers to in this volume, helping make you want to keep reading the arc.
Some interesting twists within it, and some interesting one off and newly introduced recurring characters here, a couple played more straightly as annoying, compared to the Twin Dilemna where the twins are annoying, but aren't necessarily played straight as such.
The Doctor is in good form here, showing a bit of a mellower side compared to his first season, and so more likeable for me.
Peri though, while reasonably proactive, seems to be in the mindset of wanting to avoid all trouble, such that travelling with the Doctor doesn't seem to make sense for her really.
Overall though an enjoyable read.