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!
I was obsessed with the Black Stallion series. I still have the books and the movies.
The first books I can remember being in love with were the series of unforunate events by lemony snicket. I still read over them now, they are so brilliant!
My fist book obsession was with Pride and Prejudice, I remember that I loved it and I just wanted to read more novels like that! so I became obsessed with Jane Austen's books and then it was Harry Potter, Anne Rice's books and a lot more, but Pride and Prejudice by Jane Austen was my first love. :)
Ella Enchanted. Ah, good memories. Actually ruined the first one because I read it so much and bought a second book... that one is also worn but loved.
Peter Pan/Peter and Wendy. Fell in love with the Mary Martin stage version on video at age two. By age 4 I'd taught myself to read well enough that I read the original book, and loved it even more. By age 7 I wrote a 9 page essay on the differences between the books and various movie adaptations, and how none of the movies come close to capturing what the book was trying to say....
I got obsessed with three series. the two obvious ones, harry potter and nancy drew, but the third was a series called hermux tantamoq. i loved those books.
The first book I was every really obsessed with was a book called They Cage The Animals At Night by Jennings Michael Burch. It's an autobiographical story about Jennings as a boy of about 8, who was left at an orphanage and bounced from foster home to foster home, never in one place long enough able to make any real friends. His only friend was an old stuffed animal that he named doggie that was his only constant companion for most of his childhood. It's a totally heartbreaking book to read, to realize what he went through as a child. I cry every time I read it. It's horribly depressing at first until you realize that despite all the loneliness and lost feelings this kid went through..he grew stronger for it and had a happy ending.
I first read the book when i was in 5th grade I think, and several years later the author came to town for a book signing that I told my mother I HAD to go to and meet Jennings. There he was, signing my tattered copy of the book...with doggie sitting on the table next to him. I can't tell you how happy I was to see doggie was still with him and I got to give them both a big hug!!