first book, probably "captain underpants" when i was 8. embarrasing name, i know, but i really liked it.
although i've obsessed much more over other books since then, when i started reading the daniel x books, i broke my record for not using anything electronic. about 19 hours, 56 if lights and stuff that helped me with my reading don't count.
The Tiger Rising by Kate DiCamillo.
I read it in 5th grade I think and I've read it 5-7x since then. I feel like it had so much meaning in it without really trying. I think thats how books are supposed to feel sometimes though.
I think the first book I became obsessed with in that I spent all my time indulging in the fandom afterwards was Bridge to Terabithia, which I had to read for school in sixth grade (it was around the time the movie came out, and we took a field trip to see it and then had to write a compare-and-contrast essay for English). Most of my classmates hated it, but I thought it was sweet, happy, and beautiful.
I stayed obsessed with it through the end of sixth grade and into the first few moths of seventh grade, when I read The Outsiders and became obsessed with that until the end of eighth grade. In freshman and sophomore year, I kind of stopped reading, but then over the summer I discovered The Vlogbrothers, and ultimately Looking For Alaska. I stayed riled up about it for a couple of months, but having just finished Paper Towns, I've been pretty obsessed with that as well. It's hard to decide which of the two is my favorite. :)
Anyway, to answer the question, the first book I ever fell in love with was Bridge to Terabithia.
My first ever book obsession was definitely the Harry Potter series. Before that, I mostly read Magic Tree House books, but I was never really obsessed with them.
I totally loved The Sideways Stories of Wayside School. I refuse to get rid of them, even to this day. They were so quirky.
However, nothing will ever meet my absolute obsession of Harry Potter. I pretty much had Harry Potter everything!