I'm going to give the same answer to both these questions:
'The Hitch-Hikers' Guide to the Galaxy'
I first read it when I was13 and have never been without a copy since. It's a work of genius; hilarious and gloriously absurd, and it introduced words and phrases into the English language such as:
Pan-Galactic Gargle Blaster
The ships hung in the sky in much the same way that bricks don't
Itt is mistake to think you can solve any major problems with just potatoes
The Great Green Arkleseizure and the Coming of the Great White Handkerchief
and of course...
42, the ultimate answer to the ultimate question of life, the universe and everything.
Who couldn't love a book like that???