His Dark Materials by Philip Pullman
These three books are my favourite books in the whole world!
They follow the progress of Lyra, a girl from another world, and her demon Pan. They explore the relationship between humans and their souls, the concept of origional sin and many idas surrounding religion, death and the afterlife. And they make a damn good story too!
I've read these books at least ten times over and I never get tired of them. Pullman writes a modern-day epic that is readable and addictive and will surely become a classic.