Discworld Quote

I am writing an essay on the nature of detective fiction, and need  to source a quote from a Discworld novel.  Death is talking to someone about Mystery Novels, and how they are a pointless construct as you can just skip to the end and discover the identity of the murderer.  The question is - which book, and if you're feeling generous, what's the page and the quote?

Many thanks,

G

It's the Fifth Elephant. Sorry don't have the book on me at the moment, but Vimes is having a near death experience while crawling along the cave floor. Death, meanwhile, is reading a mystery book while waiting next to him. Death says something like, "This book has no purpose. Why would you remain willingly ignorant?"