The Doctor is hardly in The Sleep of Reason for the first hundred pages, and it doesn't matter at all.
"How can this be?" I hear you ask in unison from thousands of miles away. "The last time we read a Doctor Who novel we got so bored whenever the Doctor went offstage that we had to freebase raw coffee grounds just to stay awake!" I will tell you. It is because the original characters in about half the Who books have the inner lives of mercury-addled flatfish. They haven't a prayer of carrying a novel on their own. I don't know for certain why this is, but I have a half-assed and probably unreliable hypothesis: They're just plot puppets.
Because, see, most BBC books have about as much plot as you'd need to fill out a 140-page Target novel. Now, in your usual non-BBC Books sort of novel, this is not a problem, because there are non-plot-furthering things you can put in a book that are just as good: ideas, interesting conversations, philosophical bits, worldbuilding... even, to be relevant, characterization. But the BBC writers seem to think what's needed is more plot. They want more events! Stuff should be happening! On every page!
So they bulk the books out to 280 pages with busywork cleverly designed to resemble plot. Our heroes get captured, escape, run through corridors, get captured by someone else, escape, run some more... And it looks like things are happening, but you can have whole chapters of this stuff with no real progress at all. It sucks up extra pages the writer might instead use to make the characters live and breathe a little. In a way it encourages the writer not to make the characters live and breathe, because they must perform bizarre, unmotivated actions to keep the plot contrivances going.
Plot puppets. Clacking and stumbling with each inexpert jerk of their strings. You can almost smell the sawdust. The latest poster child for this kind of thing is The Deadstone Memorial, a book in which absolutely nothing that any character does makes any sense at all.
When the regulars don't turn up for a long stretch, in one of these books, we get bored. We skim, looking for the Doctor, or even Fitz if we're desperate, skipping the rest. Not because the regulars are better characterized. But it seems like they are. They're already familiar from other books, and some of those books are better written. We fill in for the writer's deficiencies with the awesome power of our fan-brains... like looking at a picture of a vase, and seeing faces off to the sides. Even if all the books were weak, the writers' different takes on the characters would add up, in our minds, to something greater than the sum of its parts. Doctor Who books can get away with weak characterization because they're propped up by the efforts of all the other books in the series.
The Sleep of Reason doesn't need props. Martin Day writes characters even a normal sane non-fan person could care about. If you want to see how he does it, just look at the first chapter, entirely devoted to the introduction of his main supporting character, Laska Darnell. It's probably the best writing in any of 2004's Who books, which is a good thing. Day's writing about mental illness here, and...
...And here is one of the places I tripped up earlier. I kept trying to write things like "because it's a serious subject" or "because it should be handled tastefully." Which are just wrong. I think, given a skilled writer, that any subject matter can be handled by any medium or genre, from serious literature to light comedy to cartoons to pop music. (My CD collection actually contains a couple of catchy tunes about self-harm.) I finally realized what I wanted to say this morning, when I read a column by Roger Ebert where he briefly discusses whether a movie has "earned the right" to use a certain bit of imagery. Which is a good way of putting it. The Sleep of Reason earns the right to deal with mental illness, because Martin Day has put such care into it. A book with the same characters and plot, but written by Terrance Dicks, might have trivialized the subject the way The Eight Doctors trivialized drug abuse. On the other hand, a Dave Stone book, no matter how weird or farcical, would probably still have worked.
Beyond the quality of the writing—and maybe even the reason for the quality of the writing—is the fact that Chapter One really is entirely devoted to introducing Laska. Typically the Who books jump from scene to scene, and character to character, every few paragraphs like unmedicated ADHD sufferers. For The Sleep of Reason's entire first chapter the focus stays on Laska. Day has the discipline to stay with her history and her POV, and he keeps this kind of thing up throughout the novel—large chunks are in Laska's POV, and much of what's left is first person narration from the diary of Dr. Christie, an earlier director of the institution where Laska currently lives.
I think that's the crucial point—The Sleep of Reason works because Martin Day has given the characters the space and the attention they need to come alive. He doesn't crowd them out with a hugely padded plot, and he doesn't shortchange them by jumping to another point of view before we can get to know them. It's a subtle distinction. You wouldn't notice a difference between The Sleep of Reason and the average Who novel if you just picked it up and glanced through it. But it makes a big difference when you read it, and I wish the Who novels would follow its example more often.
An extra observation that I wasn't sure where to put: One happy side effect of all the characterization is that the Doctor is less stupid than usual. You know what I mean. It's part of the plot puppet syndrome. The Doctor makes stupid mistakes, dragging out the time it takes to solve problems, just so the book can make 280 pages. Alienating potential allies with weird unsupported claims and insane behavior. Going back and forth to places he's already been to pick up information that he should have gotten earlier. That kind of thing. The Doctor doesn't need to be stupid in TSoR, because Day doesn't need to pad out the plot. This time he's gone to the trouble of working at the Retreat for a while, to build up trust with the locals before the craziness starts. He only tells people as much as he has to. (And when he does start talking about aliens people don't automatically believe him, as has occasionally happened.)