First book you couldn't put down?

This question was inspired by MrMeltedCrayon on a thread talking about the book Siddhartha: Which book do you remember reading cover to cover, because you couldn’t put it down? Also, if you would – please tell us how old you were at the time and also if you have gone on to reread the book later in life.

Mine was Mrs. Frisby and the Rats of NIHM by Robert C. O’Brien when I was in the 5th grade. I have reread it once since then, and somehow still have that same copy in my library.