Crumbs, that's a tricky one as it was likely one I read as a nipper.
I do trace my love of fantasy and sci-fi back to The Boy With The Bronze Axe, which was a great historical book for kids. From there it's a clear line through The Lion, the Witch and the Wardrobe to The Hobbit. I should also flag Star Wars up as a big influence - I was hungry for more of that so drank books down, on long drives over the holidays, I'd get through a book a day. I remember there was a book offer for schools and I got quite a few that way, especially the John Carter of Mars books. I trawled through secondhand book shops grabbing armfuls of whatever took my fancy.