This wasn't quite the book I expected. (I thought I had read somewhere that this was the one where various different versions of canonicity collide; obviously not.) This is an alternate timeline where the Doctor never left Gallifrey, and neither did the Master (here renamed the Magistrate); both ascended to high office among the Time Lords. The Doctor is brokering a peace deal between the Sontarans and the Rutans, but meanwhile Omega is trying to break out of his anti-matter universe using the Doctor's body; so it's a combination and re-orientation of The Three Doctors, The Invasion of Time and Arc of Infinity (Hedin is the only other TV canon character in the story) with some very small bits of The Deadly Assassin and The Five Doctors. There are also a number of nods to Stephen Baxter, which is mildly amusing.
While I liked the overall idea, and numerous details of the scenery, I wasn't so sure about some of the plot. I felt that the Gallifrey audios managed to balance the idea of competing factions in Gallifrey, powerful external forces and rogue Time Lords rather better, and without having to invent a whole new continuity. One crucial point is that the Gallifreyan security system really is unrealistically poor, even for the sclerotic Time Lord society: there is little sense of urgency from the President and High Council as the body count rises in the corridors of the citadel, or when important visitors start going astray, and the Watch's investigations are astonishingly incompetent.
However, the interactions between Time Lords and the rest of Gallifreyan society are well done, and so is the depiction of Omega's universe and its limitations. There are also some intriguing hints about the Doctor's own lost past, and his capacity for loving women of his own race. (And of course it's impossible to know which Doctor we are dealing with here - Paul McGann without the wig, perhaps?)
Fails the Bechdel test, I'm afraid. There are few women characters - Omega's unnamed wife, Larna who is the Doctor's quasi-companion, and Larna's maidservant; the first of these never meets the other two, and the only direct speech interchange between Larna and her maid is about a male visitor.
Otherwise, not bad, but not a classic either.