Sourcery isn't my favorite Rincewind book, but the ending is what really makes it worth reading. 

You gotta love Rincewind's talent for knowing immediately when to run away from a situation. He has insights even he isn't aware of (which is one of the reasons I love him so much). 

One of my favorite lines in the book, which just demonstrates Terry's talent for writing, is "Nothing moved except the shimmering air, hot as a stolen volcano"

I didn't particularly like the middle of this book, but the last 50 pages or so are absolutely incredible.

Even though Rincewind has to be dragged kicking and screaming into being heroic, it absolutely melted my heart to see him shove Coin behind him to protect the boy, and then immediately get clocked by the staff. This is followed by him saving Coin yet again in the dungeon dimension, effectively dooming himself.

Rincewind's greatest talent is knowing when to run away, but his greatest character trait is heroics under pressure.