Quite a hard one for me to rate objectively as such here - between a 2 and a 3, but a lot closer to a 2 for me, and potentially the worst book for me that I've put onto Goodreads, or at least in the mix with Cthulhu attacks book 2, and the strain book 3 (and I tend to not finish the truly dire books, so won't even rate those :) ).
The issue isn't necessarily the writing or arc though, it is the out of character personalities that do it for me.
Compared to most 2 episode stories that have been novelised, this one feels more padded, and padded with lots of internal dialogue that really doesn't marry with my views of the characters. The Doctor comes across quite misogynistic in this one, and the relationship portrayed here between him and the Tegan, if was the way it always was, would make you wonder why Tegan returned in Arc of Infinity, and why the Doctor accepted her back on board.
This also leads to the scenes with them in being coloured by their personal views of each other, just making it really feel like it isn't Doctor Who. The Master seems to be similarly affected, as with the one off characters, making it hard to enjoy for me.
Turlough isn't quite as bad, but still not ideal in inner dialogue, but at least his scenes seem more true to him, and he ends up taking the damsel in distress role for the story, but does it quite well and in character, as even after the events of Enlightenment, he is still something of a coward at times.
So overall, on a personal basis, closer to a 2 star, but that is due to the characterisation being off what I think it should be, not because it is necessarily a bad story.