My first true literary obsession was the LotR trilogy. In fourth and fifth grade, my friend and I were really into those books and would talk about them whenever we had an excuse to. No regrets.
I don't know whether many people know these books, but I loved the Sheltie books. They were about a girl called Emma and her little Shetland pony called Sheltie. I also loved The Saddle Club but the first books I became COMPLETELY obsessed with was Harry Potter but that didn't start until I was twelve. :)
Harry freaking Potter! :D
I didn't really read much before I discovered Harry Potter. They sort of got me into reading.
My first major, major obsession was Harry Potter, actually. I got the book for my sixth birthday, read it faster than anything I'd ever read before, then watched the movie in theaters about two weeks later. The next year, I dressed up as Hermione Granger for the first (but most definitely not the last!) time, and had a Harry Potter themed birthday party, thus solidifying my move into the world of nerdity.
Before that... I really liked the Magic Treehouse series in kindergarten. I wouldn't call it an "obsession," though.
Magic Treehouse was amazing!!! Those books are the source of so much random knowledge... like what to do if you think a shark is stalking you...
I was never really much of a reader. There was one book, though, I remember reading in middle school when I was home sick for a couple days. It was the first and last time I had ever started and finished a (multi-hundred page) book in less than three days. It was called The Transall Saga (I think). I don't even remember who it was by. It just thrust me into a new world using a plot device I had not read before. It contained useful survival knowledge and melee combat basics. It had a slowly unfolding mystery while maintaining action to keep the story moving swiftly along. And this isn't the tiresome, specific action that would cut every Salvatore book in half if it were removed (nothing against Salvatore, I have a special place in my heart for his books, but...)
I really want to get ahold of this book soon and read through it again to see how much I remember and how much my comprehension has changed since then.