I'm thinking ...
So many books, how can I pick only five?
Definitely got to have The Fifth Child by Doris Lessing, because I hadn't read anything like it before and it shocked me, I was pregnant at the time, so it scared me, but I was also fascinated .
I loved, loved, loved The Magician's Nephew by C S Lewis, because it made everything in TLTWATW fall into place.
One of the Bill Bryson books - not sure which is my favourite ... probably Neither Here Nor There.
A Brave New World, I loved the concept of this book, we read it at school and I'd hated everything we'd been given up to that point, but this was completely different and the first scifi/futuristic thing I read.
Anne Frank's diary - my heart aches every time I read it.