This Doctor Who book was not so original in its storyline, but it gets brownie points for still making it exciting. Seriously, I should some day count how many stories have alien possession in them. However, in this case while it was a major part of the story, it wasn't the only thing going on.
The appearance of the Sontarans was a surprise, since I didn't read the synopsis. It added an interesting twist to the story. Some central characters were Sontarans, and they weren't portrayed in a completely negative light, which was nice. The author captured the species well, though the resolution to the conflict between the leader and his subordinate felt odd and a bit out of place. I suppose the author wanted to add something of Sontaran culture into the whole thing.
Our main positive characters were humans again. The family running the local hotel were well-rounded, not one-dimensional. What I did find strange was the computer thing with Vienna. It just didn't seem like something the Doctor wouldn't know. If he did know it, then it wasn't communicated well. The Doctor's companions sometimes do know things better than he does, but in this didn't seem like something that would fit that. He is often socially challenged, but not computer challenged. The boy's thing did fit.
An interesting character was Mr Smalls. I thought that was a reference to Bill O'Riley. Would he do the same thing? Probably. Hopefully, anyway, if he's not just all talk.
Overall, the book is a good, fast read. It did make me wonder how they were going to resolve the crisis, so that's why I gave it 4 stars. It wasn't completely predictable. But seriously. No more human possessions, please!