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!