I went to a reasonably good school, though I think I hated the headmaster just as much as he hated me. Time and again I come back to the library as where I got my real education, and The Way of Terry Pratchett is this: you go through the very, very top of a very big library and you read every last book, which effectively is what quite a lot of my adolescence was made of.

That meant I’d read all kinds of books, and things stick, all contributing to the great big compost heap from which the roses grow…

Why did you become a journalist?
When I was in school I wanted to be, among other things, a writer. I was sufficiently clever to read some books about it, and I learned that the chances of making any kind of living writing fantasy and science fiction is remote… but if I got a job on a newspaper they’d have to pay me every week.

I went round to the local paper to find out more, and they told me they had a job right then! Well, journalist jobs were hard to come by, and I didn’t want to miss out. I ran away from school, and I told my parents I’d been given a job. They were pleased that I’d found work that didn’t involve getting my hands dirty.

What did you like about being a journalist?
The first day I was a little journalist I was sent out with a real journalist and I saw my first dead body, and it was an extremely dead dead body. When you are a journalist, you see the world, the nasty bits and the good things. You see democracy in progress or no progress whatsoever. You learn about people and get into people’s worlds which you’d never get into otherwise. It turns a boy into a man, and it grew me up.

What did you read when you were a child?
I wasn’t a reader until I was given a copy of The Wind in the Willows when I was 10. I’d never seen anything like it, and I read it straight through, thinking: this book is seriously weird! The rat, mole and badger all change sizes constantly. A toad drives a motor car, and the horse that pulls the caravan is the only animal in the whole story who can’t talk. I didn’t understand – and I fell in love with that, with the suspension of disbelief the story demanded.

It was just a few years later I read The Lord of the Rings in a single 25 hour sitting… By then I was mad for fantasy, reading with the kind of speed you can only manage in your early teens.

Why do you think fantasy so accessible to both children and adults?
You start with the fairytales and the fairytales segue way up as you get older and you start realizing, if you are growing up, that fairytales aren’t exactly the same as what’s really going on and what’s happening around you. And if you’re paying attention, you ask why and look for more stories to help find out.

What is the appeal of fantasy and science fiction for storytelling?
We like the idea of “What if?” Because even back to Neanderthals you had to be inquisitive and think about the future being different from what it is now, otherwise you’d be toast. All the time the human race asks “What if?” Sooner or later it happens that someone will say that’s the future. We make these things up, it gives us a vocabulary of ideas for the world being not exactly like it is now.

Which of the books that you’ve written is your favorite?
I don’t know about “favourite,” but the four Tiffany Aching books, starting with The Wee Free Men, are very close to my heart.

Lots of girls – and some boys – say they like Tiffany because she is real, and she gets on with things and doesn’t complain until it is necessary and does the job that is in front of her all the time. Some of these notions seem old-fashioned these days, but I got them from people I knew when I was growing up. And of course, lot of Tiffany’s story is set in Chalk country – the countryside I lived on then and live on now.

What advice do you give to children who want to be writers, and how does it differ from what you tell adults who want to be writers?
I tell aspiring young writers to keep a diary and keep it going whenever you can. I wish I’d done that. I’ve done it occasionally and I fall out of it. But it should be a proper old time diary including, “What I thought about when this happened” and “This is how I felt that happened…” Remember who you’ve been, and who you’ve met, and all the funny faces you’ve seen.

I tell adults that the nice thing about being your own boss is that you can’t be sacked.

• Terry Pratchett’s new book Dragons at Crumbling Castle is published on 11 September 2014.