Both cutting the cake and eating it

Neal Stephenson's Anathem is a typically robust brick of a novel, 937 pages packed with action, maths and top facts. It was a Christmas present, though the weight of thing put me off starting it until my long flight out to Florida.

At first, I thought it was running along the same lines as my great favourites A Canticle for Leibowitz and Riddley Walker: the people of a post-apocalyptic Earth struggling to put the world back together, making sense and science from the fragments left of the past. For the first 200 pages that's exactly what it is, detailing young Erasmus' life in a Concent, caught up in chores and philosophical discourse, and cut off from the world and his family outside.

But there's quickly hints of something going on in that external world which will will affect the young scholars – and might even lead to a fourth great Sack of the concents. Erasmus is soon on a peregrination into the dangerous exterior, trying to make unravel what's happening.

Without giving too much away, the quest and mystery are suitably thrilling, while allowing much discussion of Big Ideas. A lot of that discussion – on mathematical proofs, on etymology, on perception – is engrossing.

Admittedly, one chapter is more than 100 pages of one great conversation over dinner. It's broken up with trips to the kitchen (where people comment on the conversation), and notes on the food, but it left this reader rather weary. Especially since it's right in the midst of some very exciting stuff involving explosions and – hooray! – unexpected ninjas.

But generally, what makes this – and Stephenson's work as a whole – so compelling is the deft mix of the action and theory. There's the dizzying wheeze that our brains, by being able to imagine other worlds and circumstances, work at the level of quantum uncertainty – that we flicker between possible Narratives and even physically rewrite the past.

(See also the Telegraph's recent list of the top 10 weirdest bits of physics.)

There's a nice idea on page 102 that becomes integral to the plot: there are no new ideas, and the order's job is not to invent new philosophies but to tend, nurture and preserve the wisdom and insight (“upsight” in the book) of the past, like gardeners.

I also thought Stephenson's invented lexicon – the glossary lasts for 19 pages – might lose its appeal pretty quickly, but it's nicely woven through the story. Usually, we learn the meaning of a word just in time for it to become pertinent, so that the invented etymology is a kind of foreshadowing, adding layers and depth to the plot.

It's a gripping adventure, and there's loads I'm still picking over – the plot, its ramifications, even just some of the top facts. It's a geeky, lively, often funny book, full of great characters and moments. And it's got the best, most satisfying end of any of Stephenson's novels.

It's just a bit too long, with a wearying intensity that means it sometimes feels like homework – or, perhaps, as if we're part of the Concent ourselves. But then Stephenson's recent Baroque Cycle – which I loved – also demands a great deal of effort from the reader. This is not an author for the faint of heart; but he's also massively rewarding.

(See also Stephenson's lecture on the geeks inheriting the Earth and my thoughts on his novel, Cobweb.)