It is again a very large book, and there is again a preoccupation with appropriate language for the setting, which preoccupation, as in the Baroque Cycle, becomes less strictly observed as the book continues.
The basic setting is that, on the planet Arbre (which may be an alternate Earth), philosophers and scientists, rather than monks, are walled up in monastery-like institutions called concents. Religious practices take place in the outside world, not in the concents. Extreme conservatism and ritual characterise life in these institutions. Stephenson engages in considerable discussion of logic, cosmology, mathematics, set theory, and philosophy, which make the start of the book rather slow, and a number of the complex concepts are consigned to Appendices.
The pace speeds up when visitors from another cosmos appear in a spacecraft that looms rather threateningly, and the story sets off in a new direction.
Eventually, parallel worlds become the theme of the book, and provide its (quite convenient) denouement. But the journey to the end of the book is an entertaining mix of mind-bending concepts and well-described action. Above all, the book is crammed full of cool new ideas.