Wolfsbane does something that not a lot of Doctor Who stories do: explore the question of what would happen if one of the companions got left in a time period when the Doctor and the others take off again. This means a bit more in the era of the Fourth Doctor, when the TARDIS wasn't as controllable as it became in later seasons. However, Rayner gives him a bit of control to make the story feasible. Wolfsbane is one part werewolf horror novel and one part comedy of manners in the English aristocracy. The meeting of the two is sometimes ingenious and sometimes jarring, but always at least slightly interesting.
One of the strengths of this book is the beautiful characterization of the regular cast, at least for the most part. The Fourth Doctor, notoriously hard to "get" in a novel (Tom Baker's performance on television is hard to catch in prose form), is actually pretty good. He's moody at times, always quick with an absurd witticism when it seems appropriate. The escape from the scientists' lab is one beautiful sequence showing the Fourth Doctor at his best. However, there are times where he seems a little too distant. He barely seems to register the fact that they've left Harry behind, and Sarah's understandably angry with him. However, while the Fourth Doctor is decent, the Eighth Doctor is wonderful. This adventure takes place, as far as he is concerned, during his 100 years stranded on Earth, with his TARDIS slowly growing back from a featureless blue box and with no memories of who he is or what he was (explaining his not knowing Harry), Rayner captures him beautifully. Especially noticeable is the need to fight evil wherever it is, even if he doesn't quite know why he must do so.
However, this is not either Doctors' book. Instead, it is Harry and Sarah's. Harry is the perfect person to be stuck in a "proper" English household, always trying desperately to do the right thing, use the right spoon, and not intrude on private moments. He even has the language down, with "very good of you...but, mustn't intrude - house of mourning and all that." He and George Stanton make a wonderful pair for dialogue when they're in a scene together. Harry's comfortable allowing the Doctor to lead the way in the investigation into what's going on, but he keeps having doubts that this is *his* Doctor and so has trouble trusting him at times. Harry blunders along, trying to avoid the passes that Emmeline Neuberger, a German woman who appears to be looking for a man so she can stay in England, and Harry would make the perfect match. He shows a wonderful mixture of intelligence and naiveté that is the hallmark of Harry. Sarah, on the other hand, is determined, willful, and the perfect embodiment of what Lis Sladen brought to the role. She is independent, but understandably scared when something really awful happens to her. She's a wonderful character that springs off the page whenever she's on it.
The plot is a smorgasbord of stuff, from werewolves to Arthurian legends, along with the aforementioned British aristocratic comedy, and the tone of the book is slightly off at times when Rayner doesn't seem to be able to control the mixture. The scene transitions are jarring at times, from the dark foreboding of Sarah's search for the truth about Harry's death to the Stanton family and their stiff upper lip. While the overall atmosphere is very well done, it doesn't always fit. The other problem is one that may not be a problem for some. This is a Doctor Who novel solidly in the horror/fantasy genre. There is no rational explanation for any of the things that happen in the book. Instead, all of the explanations are magical in some way, either nature spirits or the werewolves themselves. If you're a Who fan that can't stand any kind of "magic" in your stories, you will hate this book. Personally, I'm on the opposite side of the fence, but I think Wolfsbane went a bit too far toward the magical side.
That doesn't take away from an otherwise excellent book. The characterization, aside from the regulars, is rather spotty and thin, but the intricate plot and atmosphere shine off the page. While reading, the plot doesn't really seem that intricate, but I loved how Rayner sometimes related the two different timelines; Sarah finds out something in her investigation and then we see the real story of what happened for that event with Harry. This often makes up for the jarring transitions. Especially atmospheric is the story of the Night of the Long Claws, where the Germans rounded up the werewolves, starved them, and then set them loose on a town outside Munich on the night of the full moon. This chapter, told by the wolf, is exceptional, and well worth the read by itself.
So, Wolfsbane does what it sets out to do: tell an entertaining story with a good use of the Who regulars. While it's not a standout, there are enough excellent bits in the story to definitely make this worth a read.