Me, it's the #CYOA / #ChooseYourOwnAdventure pocket books. Special mention, CYOA #5 “The Mystery Of Chimney Rock” by Edward Packard. I think I was 4 then, when I saw the book on my father's shelves; published two years before I was born. LOL.
I got hook with CYOA and started writing my own CYOA stories. I wish I still have the notebooks I used for those stories so I can put them online!
I also remember how I let my classmates give feedback about my own CYOA, even explained to them how it works. After they've read all my works, it was only then I introduced them to the real CYOA books. (Sorry Edward! But, they probably won't read my own stories if I introduced the real CYOA first. bows) I also thought of submitting my works to CYOA!