Feersum Endjinn

Feersum Endjinn is a science fiction novel by Iain M. Banks; unlike most of his science fiction, it does not feature the Culture. One of the most notable characteristics of the book is that one part in four (and the title) is written in the voice of a character who spells phonetically (eg "I 1/2 2" for "I have to").

Warning - Wikipedia contains spoilers

The book is set on a far future Earth where the uploading of mindstates into a world-spanning computer network (known as "the Crypt") is commonplace, allowing the dead to be easily reincarnated (though by custom, only a limited number of reincarnations are allowed). Humanity has lost much of its technological background, partly due to an exodus by much of the species, leaving behind only those unwilling to leave Earth. Unknown to most of humanity, the Solar System is drifting into an interstellar molecular cloud ("the Encroachment"), which will eventually dim the Sun's light sufficiently to end life on Earth. However, the Ancients have left behind a device (the "Fearsome Engine" of the title) to deal with the problem; the book follows four characters who become involved in the attempt to activate it, with the narrative moving between the four (who do not meet until very near the end) in a strict rotation.