This book was the first Doctor Who Original Novel I've ever read - I've read the "Sea Devils" book a few years ago starring the third doctor - but that was a novelisation of the television story.
The great thing about these novels is that there is no budget for special effects. Imagine the effects it would require to create about a hundred different types of mythical creature, or to create an impossible unfolding chest in which Fitz gets trapped (oops - spoiler there!), or indeed the sheer magnitude of the whole of san fransisco being sucked into a scar in the space time continuum...
The story begins with the doctor trying to convince an alternative brown-haired version of his companion sam to come with him to put right what he has accidentally wronged. He slowly gains her trust, only to lose it again and gain it back many times. You see, theres this thing called "The hunt" which changes Sam's "Biodata" - which changes her history, and future and memories... which makes her doubt whether things in her life have actually happened.
I especially enjoyed Faction Paradox and the little boy. There was a small boy who was very cheeky, but also very clever. His sole purpose was to medal with time and create paradoxes. An example of this was when the doctor was creating a scientific fluid to summon the boy - the boy was actually in the room trying to put him off conjouring him - if he'd have succeeded there would have been a small paradox.
Anyway, I'm giving way too many spoilers away... but if you liked the movie starring McGann's eighth doctor, you'll like this as it links in very well. A word of warning though, it starts well and ends well, the middle is heavy going, but stick at it, you won't be disappointed.