The Discworld series. I can't remember which I read first (possibly Reaper Man?), one of my parents had a few and I picked one up at a very young age.
I remember buying my very first book out of my own money at about 11, it was Witches Abroad (specifically went to buy a new Discworld book) and I read it so many times it fell apart and I had to buy a new one ha ha!
At the time I only understood some of the jokes but not many, got hardly any of the references but enjoyed the stories and the humorous way he wrote.
My sisters ex, bless his lovely socks, once stood in line for 2hrs to get me a signed copy of The Wee Free Men when my boss refused to give me the time off to go myself. I was gutted I didn't get to meet TP but will always have a special place in my heart for DsisEx for going for me and I treasure my signed copy.
It would be the thing I took out of my house if it were on fire 
They've given me a lifetime of happy reading, taught me interesting tit-bits, science facts and how important the distinction between Apes and Monkeys can be 
Every time I read them I understand a reference or joke I hadn't previously got before.
Just lots and lots of love for the world and characters Terry Pratchett created especially Granny Weatherwax and the Watch. Actually just all of the characters 
Was genuinely sad when he passed away in a way I'd never felt for a stranger before.