'Transit' is a rarity, not just among Doctor Who books but among books in general; it's a book that keeps getting better every time you read it. Ben Aaronovitch, who I previously described as the creator of modern Doctor Who, wrote a novel that works on multiple different levels. It's primarily a cyberpunk take on the classic Doctor Who thriller; Something Creepy(tm) has gotten out into the transit system due to explorations of places Man Was Not Meant To Go(tm), and the Doctor has to banish the monsters back to the spaces beyond. But at the same time, it's a brilliant exploration of the Doctor's effect on history, told through two simultaneous and intercutting plot threads; one that plays almost continuously in the background, exploring the knock-on effect of a single intervention of the Doctor's, and the other that builds over the course of the book, showing how his interaction with humanity has produced Kadiatu Lethbridge-Stewart. Aaronovitch is genuinely thinking about his theme and subtext, working to produce something that's more than the sum of its parts.
The primary plot, while nothing we haven't seen before, is proably about as well-executed as it's ever been in Doctor Who. The STS virus, named by the Doctor with perfectly appropriate whimsy as "Fred", is an interestingly alien alien, with goals and motivations and planning styles that feel genuinely like something not of this universe. Unlike the vast majority of pseudo-Lovecraftian monsters that populate Doctor Who (and most genre fiction), Fred seems authentically unknowable, not merely a moustache-twirling bad guy that happens to drive people insane when they look at it. And the Doctor's solution, concealed beneath layers of subtlety by the author, comes as both genuinely surprising at the time and brilliantly thought out in retrospect.
(It does take a bit of retrospection, of course. One of the things that's most noticeable about 'Transit' is the prose style; form follows function in this book, as the theme of "transit" encapsulated in the title is incorporated into the way the story flows. The sentences are staccato bursts of text, the scenes are short, and there's frequent cutting from place to place and character to character. We are constantly in motion in 'Transit', and there's really no point at which Aaronovitch pauses to let the slower readers catch up. This is a book that almost demands re-reading.)
At the same time, 'Transit' functions as a sequel to the Troughton-era story, 'The Seeds of Death'. Back in the 60s, when Doctor Who was primarily seen as a kiddie show with spooky monsters for the Doctor to beat, the Ice Warriors were nothing more than that week's bad guys, and the Doctor consigning their invasion fleet to a one-way trip to the heart of the sun was just that week's gimmick for defeating them (conveniently off-camera, so as not to have to pay for any model effects.) But 'Transit' shows the way that this tiny skirmish was just the first battle in a much larger and more brutal war that transformed human culture, and the way that the Doctor's casual interference and refusal to stick around to deal with the long-term consequences has had major ramifications for the whole human race. The post-war society of 'Transit' is never lingered on, of course. Nothing in 'Transit' ever is. But it permeates the entire novel.
And then there's Kadiatu. Years later, of course, the TV series would pick up the general idea behind the character, dust it off, mix it with some bits of Benny that were left lying around and call it "River Song"...but here, Kadiatu is something far more interesting and dangerous. She's a human response to a Time Lord poking around into their affairs, but she's nothing so simple as a living weapon created by an eyepatch-wearing band of supervillains. Kadiatu is presented as something quite possibly necessary, someone who can operate on the Doctor's level without being defined by him. She's not the Doctor's lover or his sidekick or his partner or his assassin. She is our representative to the greater universe, a higher being who can operate on the plane of the Time Lords and the Daleks and the various time-active powers with humanity's interests paramount. Which raises all sorts of interesting questions as to whether or not that's true of the Doctor, and whether we can really trust someone so alien even if he does claim that human beings are his favorite species, and whether Kadiatu's struggles with her inbuilt propensity to violence make her worse than the Doctor, or just more human...all of which is pretty heady stuff for a novel that's been consigned to a ghetto even within the ghetto of science fiction. If you ever wondered why I defend the Doctor Who novels, this is Exhibit A.
And of course, there's so much more to talk about. The subtle jokes in the glossary at the end, the casual brilliance of Ming as a character, what exactly the book was that Benny was trying to decrypt...as mentioned, this is a book that rewards re-reading. And lucky us, it's so damn amazing that you can't help wanting to read it again, too.