Sourcery is my favorite Discworld book.
After 'The Colour of Magic' and 'The Light Fantastic', Rincewind became my favorite character in the 'Discworld' series, and I wanted to see more adventures of him. Then I found 'Sourcery' in the library and rented it out. And it remains the best Discworld book I ever read.
Pratchett calls it his first 'serious' Discworld book. A wizard passes his staff onto his son but puts himself in the staff on purpose in the process, cheating Death. So this 'Sourcerer' has become a powerful Tyke Bomb
who comes to Ankh-Morpork and turns it from a lively, buzzing city into a quiet, practically deserted dystopia. Meanwhile, Rincewind has escaped the whole thing, having been the first to notice the warning signs from the rats, mice and gargoyles, and is dragged on a wild goose chase by Conina, the daughter of Cohen. The scenes in Al Khali are just played for laughs (like the "Reading stories in a harem? It will never catch on!" joke) but then the Archchancellor's hat turns evil...
. In any case, there are some memorable scenes in the book, particularly in the third act where Rincewind is sleepwalking and building a tower, because of the wizards' natural instincts to, and his reaction to the library when he tries to go home. In fact, the book has a surprisingly heartbreaking ending; it has some of the saddest moments in the entire series, in fact! But it's exciting, and has funny, sweet and awesome moments in equal measure!