Despite not featuring Unseen University or the witches of the Ramtop Mountains, this is my favorite Discworld book.
After hinting at it in "Wyrd Sisters," Pratchett paints an engaging portrait of life in the Ankh-Morporkk Assassin's Guild. The suave, stylish, chic and, well, murderous life as an apprentice assassin is, against all logic, made sort of appealing and cool, like an academy for future James Bonds.
Then our protagonist, Teppic, is cruelly jerked back to his reality -- he's the son of the pharoah in the Kingdom of the Sun, and his father has just died. The cosmopolitan Teppic has to face what are, to him, backwards and outdated customs the rest of the world has left behind centuries ago. He's right, of course, and the mystery as to what's really happening in his kingdom spins out at Teppic tries to adapt himself to life as pharoah, and try to drag the kingdom into modern times.
Along the way, there is the ghost of his father, who mournfully watches his own body being prepared for the afterworld, a sassy handmaiden, and a mysterious and forbidding high priest. Toss in the greatest mathematician on Discworld -- not a biped, though -- a parody of Ancient Greece, and a graduate assassin turned pirate, and you've got a rollicking cast plunging towards a very local sort of doomsday.
The ending is a touch ambiguous for my tastes -- Pratchett was trying to use a light touch and went a touch TOO light for my tastes -- but overall, this is an engaging, amusing and even somewhat thoughtful Discworld novel, and one that stands alone even better than most.
By the order of the pharoah, this is strongly recommended.