I can't make up my mind, but likely Pilgrim's Progress. All of these books impacted my life as a pre-teen, and so they may have been timely. I like Dora's pick 'Screwtape Letters' as well, and also 'The Problem of Pain' by Lewis
Pilgrim's Progress - John Bunyan
The Chronicles of Mansoul -An adaption of Holy War (John Bunyan) by Ethel Barret
By Searching - Isobel Kuhn