Pyramids (written by Terry Pratchett) is my favourite Discworld novel. I haven't read all of them, but this book is really cool nevertheless.

Teppic is a young assassin-in-training in Ankh-Morpork and the son of Djelibeybi's pharao. It is a county that is very poor, because every pharao who has died gets a fsckin' big pyramid, and of course everyone is trying to build a bigger one. Of course the pharaos are also gods (just like in ancient Egypt; Mr. Pratchett is making fun of it the whole book) and have to do certain things like controlling the sunrise and the sunset. Teppic's father dies one morning when he's believing the sun will not rise. A few moments after he hit the ground from his balcony the sun rose, however.

Now that the pharao is dead, Teppic returns from Ankh-Morpork and becomes the new one. He has some problems with the godlike life style and his "prime minister" Dios. Though Teppic is the pharao, everybody is doing what Dios tells them, because he has been the highest priest for a very long time.
Though the deceased pharao didn't want a pyramid to be built for him his son wants the architects to build one that is bigger than all the others before. Pyramids are bit different in Djelibeybi than here, they store the time that has passed during the day and release it at night. That's way the mummys don't get older. ;) This new pyramid is so big that Djelibeyby's suddenly flushig into another dimension.

A few quotes:

"All assassins had a full-length mirror in their rooms, because it would be a terrible insult to anyone to kill them when you were badly dressed."

"The Ephebians made wine out of anything they could put in a bucket, and ate anything that couldn't climb out of one."

"There was not a lot that could be done to make Morpork a worse place. A direct hit by a meteorite, for example, would count as gentrification."

There is also a kick-ass dialog between Teppic and a, oh sorry the, sphinx. Teppic has to answer her riddle, which is of course "What goes on four legs in the morning, two legs at noon, and three legs in the evening?". Teppic doesn't know the answer, but after a conversation about the riddle, the sphinx is irritated and Teppic is able to pass her.

The book's very funny (e.g. "the greatest mathematician ever: You Bastard, a camel") and it is definitely worth reading if you're a Discworld fan and perhaps even if you're not.