The Neverending Story. Suspend what you think you know from having seen the awful, awful movie. The book is, IMO, a work of art.
In an act of life imitating art, I stole the book from our school library when I was in grade 4. I had just been moved schools, had no friends and adored reading and recalled having seen the movie as a kid. I STILL have my copy of that book, and I'm about to turn 32. I still regularly reread it.
Bruno and Boots series, by Canadian author Gordon Korman, were big favourites of mine at that age. I wish I still had copies of my own; even now i remember them fondly. They're about two boys (Bruno and Boots) and their mischief-making at the boarding school where they live. "The War With Mr Wizzle" was my personal favourite though as I recall there are several others, all featuring the same cast of characters.