For me, it was...

 

Fantasy: second grade, reading The Lion, the Witch, and the Wardrobe. I remember being at daycare, and being annoyed that the daycare mom was sending us all outside to play in the yard, when all I wanted to do was keep my nose buried between the pages and continue following the adventures of the Pevensie children.

 

Sci-fi (also kinda fantasy): third grade, The Warlord of Mars. I became an Edgar Rice Burroughs disciple. After I was supposed to be in bed asleep, I'd sneak down by the door, sprawl out on the carpet, and read my Grandad's old hardcover edition by the narrow band of light coming in from the hall. If I heard one of my parents come out of their room down the hall, I'd scramble back to the bed and jump in under the sheets. Then I'd creep stealthily back, needing to know what exploits John Carter would be up to next in his heroic quest to save his red martian princess, Dejah Thoris. I was so immersed in the world of Barsoom that, until maybe the fifth grade, I really thought that if I stretched out my arms to the red planet and concentrated hard enough, I could be whisked there too. But I never knew enough about astronomy to identify which light in the sky it was (my luck, I would've wound up on Venus).

 

Horror: This one's harder, because I read Scholastic collections of ghost stories and spooky tales voraciously. I can't think of a particular author, originally. I do remember the first book that actually awoke in me a kind of existential dread. It was actually a children's picture book about a boy coming home from school past a corn field, and the scarecrow starts following him home. I was reading it at our cub-scout den mother's house, the last boy waiting for his mom to pick him up as the evening wore on. I must have been very tired, and somewhat feverish, perhaps: but that night I, a naive fifth grader, first feared death. I think, sometimes, that scarecrow's still waiting for me out there, somewhere. Sooner or later, he follows us all home.