I'm going to do my best to review DOWN without making it sound much more pretentious than it is. This is made complicated by the fact that the book is indeed pretentious -- at least a little. But any description (and I can feel mine heading in that direction despite my best efforts) is bound to emphasize the high concepts being thrown around here. But it should be noted that despite everything else that is present, DOWN also contains a lot of fun adventure stuff.
I think the best way to describe DOWN is to say that it is very much a reaction to (and against) the events of THE ALSO PEOPLE, in the same way that Lawrence Miles has reacted against other Doctor Who creations (of course, you don't need to have read THE ALSO PEOPLE to enjoy this one, as everything you need to know is included). I find that I generally prefer Lawrence Miles' writing when he's playing with and subverting other people's ideas rather than trying to create something truly original. He usually seems to come up with angles and viewpoints that are skewed, stunningly creative but perfectly in keeping with the original. He does not disappoint here.
THE ALSO PEOPLE gave us a highly advanced People (they were referred to as "people" in the original, but become "People" here) who live on the inside of a massive Dyson's Sphere/Shell. They are a peaceful people, existing in an idyllic paradise, and ruled by a stupendously advanced computer that is jokingly referred to as "God". In DOWN, Professor Bernice Summerfield and two students are investigating a planet called Tyler's Folly. The back cover tells us that the planet is hollow, with some sort of creatures living on the inside. While God kept the Worldsphere in a state of serenity, the apparent controller in Tyler's Folly does not have the same benevolent motivation. Also included in the story are two representatives from the People: a student of psychotics, and a psychotic Person.
The story is told as flashbacks. The inhabitants of the outside surface of Tyler's Folly find Benny in a prohibited area; they throw her in jail, suspected of attempted looting. The story is told as part of her questioning by the authorities. Sharp-eyed readers may note some apparent inconsistencies, but these are brilliantly covered by the end. In fact, parts of the narrative are an intriguing look at parts of Benny's character, the ramifications of which I never really quite thought of before.
The book does have a few flaws. As a reaction against the Worldsphere, Tyler's Folly falls strangely flat. Miles tells us about the comparison, but what we see doesn't really go far enough to drive this home. This has the effect of making the middle of the book drag a bit.
And as I look up at what I have already written, I see that I have done exactly what I was trying not to do. Yes, DOWN is a book concealing huge ideas, but those ideas occupy a surprisingly small portion of the page-count. Most of the pages are involved in a good old-fashioned adventure story, where Benny and her two students (What's this? Original and interesting characters in a Lawrence Miles novel?) descend into the Tyler's Folly underworld on the trail of a long-missing archeologist. All the adventure stuff is merely setting up a lot of the revelations at the end. Fortunately, the adventure is an engaging one, although it does sputter a little and could have been a bit more exciting.
DOWN isn't perfect, and it doesn't do all the stuff that Miles is attempting; it still makes for fascinating reading. It takes some pieces of the Benny/Doctor Who universe (from the major to the minor) and cleverly rearranges them and adds to them in unexpected ways. But beyond that, the adventure is still a fun one, and most of the comedy pieces work.